I was on my powerbook g4 last night, surfing the internet, when it froze. I had to force quit out of firefox, than i turned the machine off. This morning when i went to start it up again, it wont boot into the os. It goes to the white screen with the apple, and has the loading circle spin for roughly a minute, than it just shuts off. The hard drive stops spinning, and the screen goes black. What can i do?