why won't my laptop come on

Does it do anything at all when you power on, like make the chime sound? Is the screen black? I'd try resetting the PMU by connecting to the AC adapter, and while the computer is off, holding down the power button for about 15 seconds. If it succeeds in resetting, the sleep light will flicker and the laptop will hopefully make a tone. Sometimes doing this "jolts" the laptop into working.

You didn't mention what screen size PowerBook you have, but if you've got a 15 or 17", then you need to make sure you have a 65W AC adapter, because the lower-powered 45W AC will not power it, and can cause the computer to behave like it's dead.

Also, I'd verify the memory. The wrong kind of memory, particularly in Titaniums (black keys), can cause the laptop to play dead. Also, try only using one RAM slot, and then the other. A dead RAM slot populated with RAM can prevent the laptop from starting up.