Which os version are you using?
Please describe in detail all you have attempted to do in order to resolve the issue. Have you tried any of the following?
Shut down your computer and disconnect all peripherals (keyboard & mouse if pertinent) from your computer. Now reboot.
If the Mac starts up normally, shut it down again and then plug in one of the peripherals (keyboard or mouse first) and start up your computer again. If it does so successfully repeat the process, adding one peripheral at a time until your Mac acts up. At that point, disconnect the last peripheral you added, reboot your Mac and search the peripheral vendor's website for an updated driver.
If no driver exists or the problem remain after installing the new driver, try a different cable or a different port on your Mac.
If none of the above works, again disconnect all peripherals from your Mac, hold down the "shift" key to start up in "Safe Boot" mode.
If the Mac starts up correctly, restart without pressing the "shift" key.
If your computer still does not start up properly, shut it down and restart it while holding down the Apple+Option-P-R keys; keep holding "all 4 keys" down until you hear the startup sound "twice."
If none of the above work Disconnect all peripherals from your computer. Boot from your install disc & run Repair Disk from the utility menu. To use the Install Mac OS X disc, insert the disc, and restart your computer while holding down the C key as it starts up.
Select your language.
Once on the desktop, select Utility in the menu bar.
Select Disk Utility.
Select the disk or volume in the list of disks and volumes, and then click First Aid.
Click Repair Disk.
(If Disk Utility cannot repair, you will need a stronger utility (3rd party) - Diskwarrior or Techtool PRO)
Restart your computer when done.
Repair permissions after you reach the desktop-http://docs.info.apple.com/article.html?artnum=25751 and restart your computer.
Remove any 3rd party ram.
Reinstall the os.
If your computer is still under warranty or you have Apple Care, take full advantage of it by letting tech support deal with your problems. It's what you're paying them for.
Out of warranty - take the computer to an Apple store or an AASP. Whichever is more convenient for you.