OS X no longer works only Windows

[ruzong]

I've Windows installed on my computer for over a year, and all of the sudden the other day when I went to start my computer it no longer boots into OS X. It acts as if it's going to, then the screen blacks out, reboots the computer and I can only log into Windows. Please help!

[Giaguara]

Hold down ALT at startup, and select the system to boot to.
if that fails, clear PRAM (hold alt-option-p-r at startup until the second startup chime, release)

[ruzong]

I can get to the screen where I can select whether to run Windows or Mac...however, when i select Mac, the computer "thinks" for awhile then reboots. I don't get the opportunity to log in.

[ElDiabloConCaca]

Does "Safe Mode" work (holding down the Shift key as Mac OS X boots)? Howabout single-user mode (command-s while Mac OS X boots)?

What happens if you boot from you Mac OS X Install CD/DVD and run a "Repair Disk" from Disk Utility on your Mac OS X partition? Any errors reported?