If you're running 10.4.8, this is obviously an older computer? It could be anything - hard drive failure, etc. Even though you're out of warranty, it would still be a good idea to take the unit into your local Apple Store or an AASP and have them check it out. If it's a hard drive problem, you could replace it yourself. The diagnosis is free - you'll only have to pay if there are repair issues.
That could mean that your system is badly damaged. First, try repairing the hard drive with Disk Utility. That by itself probably won't solve the problem, though, so this probably ought to be followed up by reinstalling the system. Your profile indicates that you are using Mac OS X 10.4.8... if that's correct, reinstalling won't be as easy a task as it would be with 10.6 or later. With more recent systems, you can simply reinstall right on top of the existing system, replacing all the system files and leaving everything else alone. In older systems, I don't believe that is possible.
Since this happened right after plugging in your machine, it also may be worth running Apple Hardware Test. It's possible that there was a surge for some reason when you plugged in the machine, and it may have hardware damage.