Fan goes on but won't power up

many iBook G4s had a problem with a bad connection to the graphics chip, a common symptom was fans going on full but blank screen and otherwise failure to boot whenever you turned the machine on. pressure on the case in the right spot will sometimes allow the machine to boot - see this article or google for iBook g4 graphics chip problem or logic board problem and you'll find many reports. I had an iBook with this particular failure - no chime, no display, just loud fan spinning.

if your machine worked before but after repair shows these symptoms, it's possible you've accidentally aggravated the problem by flexing/moving the logic board - it apparently has to do with bad solder connections on the logic board

try pressing on the case as suggested in the article I mention to see if this is related to your problem