ibook G4 powers on but no activity

After daily use since January, this laptop was normally in sleep mode when not in use. A few days ago I shut it down when traveling. Starting up, it displayed the Starting Up window but it remained there for the rest of the session - with the blue progress bar full. The rest of the desktop was there behind it, and the computer was fully functional. Then I shut it down again.

After that is when the trouble started: the chime is heard when powering up, and there's the familiar whirring sound, but the screen is black. Sometimes, after powering down by holding the power button and then restarting, I get a single tone, and there's a white light rapidly flashing just to the left of the clasp -latch in front.

The computer is definitely on. It gets warm on the underside, and the green lights on the caps lock and the num lock keys go on. I've also tried slipping in a CD, which is accepted, sort of spun around, but not played. After some time holding the eject button, the disc is ejected.

Should I try resetting something? PRAM and/or PMU?

Advice appreciated,

thanks.

G4 iBook, 12" model, OS 10.4.11