There are four general causes of this issue:
1. The computer's PRAM no longer contains a valid startup disk setting when there aren't any problems with the disk itself. This can be checked for by pressing the Option key and seeing if the drive appears.
2. The internal drive's directory structure has become damaged. This requires usage of an alternate bootable system to perform the repair.
3. Critical system files have been deleted. This requires usage of an alternate bootable system to reinstall them.
4. The internal drive has died or become unplugged. This is the most likely case if the computer took a sharp impact or there are unusual sounds coming from the hard drive's location.
any chance it was not shutdown gracefully?
power went out at some point?
whatever it is or was not, would hope a few things:
that you are not running Leopard still;
that you have bootable backup clone(s) of your drive(s) and system;
TimeMachine, not that great back in 10.5.x days but better now
Every Mac Pro can run Lion or later which is a good idea, keep Snow leopard and dual boot if you still require use of PowerPC apps
Boot from DVD or Lion Recovery or a clone of the system and repair;
Follow up with Safe Boot.
Startup Manager: How to select a startup volume
Troubleshooting: My computer won't turn on
General purpose Mac troubleshooting guide:
Creating a temporary user to isolate user-specific problems:
Identifying resource hogs and other tips:
Starting the computer in "safe mode":
To identify potential hardware problems:
General Mac maintenance: