HI and Welcome to Apple Discussions ...
Besides resetting the SMC as Barry suggested, check the hard disk.
Insert Installer disk and Restart, holding down the "C" key until grey Apple appears.
Go to Installer menu (Panther and earlier) or Utilities menu (Tiger and later) and launch Disk Utility.
Select your HDD (manufacturer ID) in the left panel.
Select First Aid in the Main panel.
(Check S.M.A.R.T Status of HDD at the bottom of right panel. It should say: Verified)
Click Repair Disk on the bottom right.
If DU reports disk does not need repairs quit DU and restart.
If DU reports errors Repair again and again until DU reports disk is repaired.
Also, you could run the Apple Hardware Test, instructions here.
http://support.apple.com/kb/HT1509
And a list of
Startup Key Combinations that might help. In particular, try booting while holding down the Option key. Starts into Startup Manager, where you can select a Mac OS X volume to start from.
Carolyn 🙂