At the end of the day, this is an extremely troubling development. Yes, there might be a way to turn this around, for now, but I'd not be confident about where this might be heading in the next few months. Let's be clear: it has absolutely nothing to do with Toast. It's entirely likely it has to do with a hardware fault, either of the Mac or the hard drive. At some point, you're either going to encounter a service bill or a replacement bill for the hard drive. The question you face is how much and when do you want to pay. Were I you, I wouldn't really bother trying to salvage your current setup.
The quickest, safest resolution to this is to remove your problematic drive, and insert an older drive you've got lying around. If the older drive works the first time, then you can fairly safely assume it's a problem with the drive. The best solution is then to get a new drive, and do a fresh install from the Tiger disks. Then you can get a external drive enclosure, put the problematic drive in that, and hopefully rescue your data off the drive through a firewire hook-up. If you have Apple Care, I'd definitely recommend you let them deal with it. That's what you've paid for, after all. If you're not under warranty anymore, then it's really a question of whether you're going to pay an Apple Store the money to play around with this thing, or whether you just pre-emptively solve the issue by installing a fresh drive.
If, however, you want to play around with this, or you don't currently have any money to spare, then I've got three possible avenues for you to explore.
Press and hold CMD (apple key) + R + P + CTRL. Then turn on the system, continuing to hold all four keys. Wait until you hear the "dong" sound twice. Release the keys. See if that encourages it to boot. If not, repeat the process, but I guess go ahead and let it dong three times. Then release. If this works for you, great. But understand that this particular situation you've described is a warning. It does mean something is going wrong with your machine or your hard drive. Expect for it to happen again until you solve the hardware problem that caused it in the first place.
If those "magic" keys fail, let's try something else. Every Mac has a hardware reset button on it somewhere. Problem is, it's in a different place on every machine. Check Apple's website for instructions that pertain to your particular machine. This little reset button could help you out if the "magic" keys don't.
If neither of these simple solutions work, then it's not really a matter of whether there's a hardware fault, but instead where that fault is. The steps below will help you decide whether you just need a new hard drive or a trip to the Apple Store:
Boot with one of your other boot disks (let's call it, "Fred") and look at your volumes through Terminal.
That is, boot with Fred and go to the Utilities folder and run Terminal. At the prompt, type: cd /volumes
At the resulting prompt, type: ls
This should give you a list of all the volumes that boot disk has ever seen. If your problematic Tiger disk (let's call him "Sam") has ever been online during an instance of Fred being in charge, then Sam will show up as one of the possible directories. If you do see Sam, check that you only see Sam. If you see a "Sam 1" or "Sam 2" or "Sam <insert a number here>", you've got a problem that's described below.
If Sam is there, type: cd
[If Sam is not there, you can stop at this point. Sam's absence means you have nothing conclusive to go on. His absence probably just means you've never booted from Fred while Sam was around. Thus, you can't use Fred to give you any information about Sam.]
Then type, cd /volumes/Sam
If you get an error message at this point, you're probably screwed. If however, the prompt changes to reflect the fact that you've successfully changed over to Sam's directory, type: ls
[Note that getting an error message after attempting to change to Sam's directory doesn't absolutely mean you're screwed, either. It could just mean that you directly copied a previous boot disk — maybe even Fred himself —*onto Sam when you first installed it, instead of doing a clean install from Tiger disks. Doing this will actually copy a "ghost" of every drive to which the old drive had ever been connected. My current boot drive, for example, lists disks in its /volumes directory that its predecessor had been friendly with several years ago, even though it had never been connected to those drives at all. Thus, when I try to change to those disks I get an error message. In order to successfully switch to an invisible drive, the current boot disk must have been actually connected to the now-invisible drive at some point.]
If you successfully switch to Sam's directory, ant typing ls gets you a listing of the root folders of Sam, you might be okay. One possible cause of such a condition is incorrect drive settings. Not every internal HD can successfully be used as a boot drive, even if they have on occasion done so. You may need to refer to the website of the drive manufacturer to get specific details about whether that drive is capable of being a Tiger boot drive (again, even if you've successfully used it as one in the past). Ensure that you have the pins on the hard drive set up correctly to be a master.
Or it may be that you have faulty wiring inside your Mac. It could be something as simple as the drive failing to receive the proper amount of power to spin up correctly. A sign of probable power problems would be the existence of multiple Sams, as described above. These "other Sams" are usually caused by unexpected power loss, and the drive subsequently coming back online.
If you don't get a listing of the root folders, you're probably screwed. But again there could be some kind of pin/power problem.
You can test for this pin/power problem by removing Sam from the Mac and re-inserting an older drive that you know to have worked in the box before, say "Jimbo". If Jimbo successfully mounts, then you know it's most likely not a hardware fault with the Mac, but with the drive itself. If Jimbo produces exactly the same results as Sam is doing now (failing to move beyond the grey screen), then you know you have a Mac problem, not a drive problem.
At this point, you're likely in a world of hurt. I'm not sure how you could go forward with the drive, after you've eliminated the issue of faulty wiring/power supply.
In the above example, please substitute Sam, Fred and Jimbo with the real, current names of your drives.