Q: Don't know what's wrong

Do you have any software discs (OS Install discs) that came with the computer when it was new? If not, do you have any other Mac OS X Install disc(s)?

Which exact iBook model is it?

You can choose from this list:

http://www.everymac.com/systems/apple/ibook/index-ibook.html

Do you know which version of the operating system is installed?

Hello,

Sounds to me like a faulty graphics adapter if nothing is being shown on the screen and the fan comes on.

Just to confirm just apply some pressure to the left side of the tracker pad when you boot up. You can do this using your palm handing it down hard on the left side off the tracker pad. If something comes up on the screen then it's the victim of a problem with the graphics adapter.