sounds like there is a problem with the hard drive. Can you boot from your OSX install disk? (put the disk in and hold "c" while booting)
go to the disk utility and do a first aid on the hard drive and try rebooting. If that doesn't work you can try reloading OSX and if it happens again then it's definately the drive.
Has it definitely continued to do this? Or did it only do it once? Because certain updates cause that behavior after installing them (spin and restart).