If your computer has a known-good installation of OS X 10.3 on there, but just won't find it, try booting with the 'option' key held down -- this will present you with a menu of all the bootable partitions available on your computer. Look for one with a little picture of a hard drive if one appears.
If this works, your Startup Disk preference probably just needs to be reset. Go into System Preferences > Startup Disk and re-select your hard drive.
Sometimes, when a computer is unplugged for an extended time and has a weak battery, these kinds of settings (startup disk, volume level, etc.) get corrupted or reset and cause this kind of situation.
If your installation is truly gone, you'll probably just see two icons: an arrow and a circular arrow. In this case, reinstalling as described above would probably be the best course of action (after, of course, DiskWarrior and/or TechTool if that's an option).