I have a custom made AMD Athlon 2.0 gigahertz PC with Windows XP Pro SP 2. I have an ATI Radeon 9200 graphics card installed. I just installed a new CPU fan after finding out the other one wasn't working. Before that, I was trying to reinstall Windows XP. My problem is that, when I turn on my computer, the screen is completely BLANK, no BIOS or anything. Also, the computer won't turn off by just pushing the power button. I have to either hold on it or unplug it. All the fans work except one small fan that seems to be connected directly to the motherboard. All the lights turn on. I can't figure it out.check and make sure that you ATX connector is connected all the WAY in.
Is CPU inserted properly?
The ATX connector is all the way in and the CPU is inserted properly but still doesn't work.It sounds to me that your PSU is failing/failed.   I've just had a failure and the 'no screen-  won't turn off' symptoms are identical.

I upped my PSU rating by 160watts to 620watts.  Working OK now. It's POSSIBLE that it is the PSU. I'll try and borrow a power supply and SEE if that helps.I replaced the motherboard upon suggestion of a relative but it still didn't work. So i got a new power supply and only after it arrived and i put it in the case did i FIND out that the new motherboard required two power ribbons(or whatever they're called) to be connected to it, and i only had one plugged in. I didn't know this because the old one only needed one power ribbon. I connected the other one, and it worked! I don't know if it was motherboard or power supply or both, but it's completely fixed now!