Why won't my iBook G4 turn on?

A friend and I put his used hard drive in my iBook G4. It worked great for two weeks, but today, while I was typing a paper in Microsoft Word and a browser was open, it randomly shut off. It made an "ink" noise like a computer would if it was suddenly unplugged. Before this, it would not work with any batteries at all, so I just ran the iBook off AC power. At first I thought that it was accidentally unplugged, but I inspected the power cable, disconnected it then reconnected it and it didn't work. I have tried resetting the PMU, I know the outlet and power adapter both work, and the iBook wasn't hot when it shut off, so it wasn't overheated. Every time I try starting it up, all I hear is a quiet buzzing noise for a second like it's trying to start a fan or the hard drive, but nothing more.

It is a 2004 iBook G4 14" 1.33 GHZ 1.25 gb RAM

What is wrong with it? Please help.