Things to do if your Mac doesn't boot

Apple - Discussions - List of things to do if your Mac doesn't boot

Modified from a post to Apple Discussions

Safe mode:
hold Shift while power on
use Disk utility to repair permission and Onyx to delete caches

Single user mode:
hold Command + S while power on
at the Unix command type fsck -fy
type again to make sure there is no error
type reboot and press Enter to get out of Single user mode

PRAM:
hold Command + Option + P + R while power on

OpenFirmware: hold Command + Option + O + F while power on
While in open firmware you can reset your video and pram settings with these commands
0 > reset-nvram
Press Return
0 > reset-all
Press Return

Installation disk one:
Put in Installation disk one
hold C while power on
Use Disk Utility to repair permission and repair disk

Target Disk mode:
user a Firewire cable to connect to another Mac that's on
hold T while power on, this will make the unbootable Mac act as an external hard drive
Back up files from there
Use Installation disk one to re-install OSX

Re-install OSX if you don't have access to another Mac:
Put in Installation disk one
hold C while power on
While selection target drive, go to Options and select Archive and install
The installation will keep all your files and settings
After installation is complete delete the Previous System on the hard drive.