Eventhough it has been said that posters in this thread have tried all suggested troubleshooting, well there is no way for all other world readers to know what those suggestions were. So here is my suggestions:
1. Reset the Macs PRAM:
http://support.apple.com/kb/HT1379
2. After the reset if it still refuses to boot, try a safeboot into safemode.
http://support.apple.com/kb/HT1455
Keep in mind that safebooting does take awhile so be patient. If after say 20 minutes safeboot does not work then you need to go to the next step.
If the safeboot is successful then reboot normal and see if it will boot.
3. If all the above has failed then you will need to boot into the original OSX install DVD that came with the Mac or the Full retail version of OSX if you had purchased and installed a new version of OSX. Then run "Repair Disk" and "Repair Permissions" from disk utilities on the DVD.
If you need steps on booting from DVD and running DU, post back and we can give you step by step.
One other thing for some of the other posters here on this thread...
You should always try to keep at least 10-15% free HDD space on your internal disk that has OSX installed. Although I have had drives with only about 2% free space and had no problems, I have also had drives with about 10% free space give me these same symptoms. Once more data was cleared from the drive, OSX was happy and the Mac ran like a champ.