I can't boot OS X 10.5.2 Server after applying the Leopard Graphics Update. The screen cycles through a few shades of blue and then sleeps after five minutes.
You may try to hold the shift while booting, the computer will start in safe mode (without pref, just delete the bad pref files if you know what they are).
If it doesn't work, try repair permission and bless the system in the terminal:
bless -device /dev/diskXsY -setBoot -verbose
replace the diskXsY with the proper name of your drive.
If it still doesn't work, try fsck command to repaire the drive. Search a little on google to see the procedure.
If it still doesn't work, reset the pram and the PMU unit of your mac. If it still doesn't work, bring it to a repair shop ;-)
Happened to me after the 10.5.2 update (have yet to install gfx update)... I restarted holding shift, as said above, but did not do anything, I simply pressed restart, and restarted normally, and it then worked fine.
Sooo...
1) Turn off PC
2) Press power, and hold the shift key until boot
3) Login
4) Restart as normal
Worked for me, may or may not work for you depending on what is wrong...