Powerbook G4 starts up, then nothing! help!

Question

when i cut my powerbook g4 on it goes through the post test, begins loading the OS, then directly goes to a black and white Darwin screen (looks like a command screen). it tells me to login, so i do, then it goes to a completely blue screen, loads nothing and does nothing. other times after logging into Darwin it will just stay there. my computer was fine until i downloaded the newest update as of Saturday, October 7th. i believe it was either a powerpc update or a security update. after downloading the update all of this began to happen. i tried doing a safe boot by holding shift but that didnt work. i also tried the route of holding command,option,p and r. this did not work either. please help!!
Powerbook G4   Mac OS X (10.4.6)   512 MB SDRAM

Answer 1

This is taken from another post from BGreg.
If you can get the Tiger DVD into the system, when you reboot, I would first try the Apple hardware test to see if it point anything out to you. Hold the option key as you reboot, choose Apple hardware test and follow directions. Pick the extended test. Does it tell you anything?
If you want to boot from the OS X DVD, using your system from the DVD, hold the C key down as you reboot. You may want to bring up the disk utility on the utility menu, and repair permissions and repair the disk. Don't know that this will fix anything, but won't hurt, too.
At this point, you can also reset your PRAM and Power Management Unit (PMU), it won't hurt and might help to fix things. Reset PRAM by shutting off the computer. Restart and press the command-option-P-R keys together before the gray screen appears, and hold them until you hear the startup chime a second time. (reference Apple note http://docs.info.apple.com/article.html?artnum=2238 )
PMU reset by turning your computer off, and pushing shift-control-option-power. Wait 5 seconds and restart the computer. (reference Apple note http://docs.info.apple.com/article.html?artnum=14449#faq10 )
Cheers Don