More than likely, the Secure Empty Trash function did not hang on you, you just got impatient (no offense!). Secure Empty Trash, depending on the amount of files and their sizes, can take minutes, and sometimes an hour or more. It is not a procedure you want to interrupt, either. Unless you're getting ready to sell the computer and are trying to erase documents that contain CIA-level secret information, Secure Empty Trash is typically overkill. A regular "Empty Trash" procedure does just fine.
When you violently interrupted the operating system by holding down the power button, your computer was probably in the middle of reading and/or writing critical information to the hard drive. Violently interrupting this process caused some level of wonkiness to be introduced into your system now, probably due to a corrupt file.
For future reference, there is nothing that you need to do on your computer so immediately and so important that you violently interrupt a vital process by cutting power to do so -- because the outcome can do exactly what you're experiencing: leave your computer useless for the time being.
At any rate, I'm down off the soap box now. The first thing you should do is boot from your OS X Install/Restore DVD, open "Disk Utility," then run a "Repair Disk" procedure. Reboot from the hard drive and try to log in again -- successful? If not, try rebooting from the hard drive again in "Safe Mode" -- hold down the 'shift' key as the system boots until you're notified that you're booting into "Safe Mode" -- and try to log in again... successful?
