|Did anything happen to it or did you make any changes between the last time it worked OK and the first time it didn't?|
Any beeps? If not (and not to be too general) it could be anything--PSU, motherboard or added peripheral. With the power cord removed temporarily disconnect the drives and remove any add-on cards you may have. Then start it up and see if you at least get a posting screen or HP logo. If you do then one of the disconnected devices must be at fault. If not, well then it must be either the PSU or motherboard.
If you do get beeps, what are they?