wont start up properly

your Mac is likely having a problem either finding your startup disk or the system software. it may be a hardware problem with your disk drive, or just corrupted OS files on the disk.

try booting from your OS install disc - hold the C key down with disc inserted while you reboot.

then after booting, go to Disk Utility (in the Utilities menu) and try repairing your disk volume. if repair is successful then reboot and see if everything is ok. or you may need to try re-installing the OS

if you can't even mount or see your internal hard drive in Disk Utility, then it's probably a defective hard drive.