Hardware or software issue?

I have an imac g5 and suddenly it would not power on. After a day of trying to get it to work, I could power it on but it is extremely slow booting up and when I try and run the disc utility and verify the disc I get the spinning ball and then the whole computer freezes. I ran an Apple hardware test and that passed. Any suggestions?

Could be a failing hard disk, or a failing power supply.

What did the 'Apple hardware test' test exactly?

Try resetting the PMU - if applicable to your model of iMac. iMac G5: How to Reset the SMU