Desktop probelms...any suggestions?

My HP desktop is acting up. When I power it up, I don't get any screen action. It doesn't seem to 'sound' the same as a typical boot up and will continue to work (fan running, hear harddisk huming) without result. I've taken it apart and reseated all the cables and cards, switched out HDMI cables, and reset the BIOS battery but no results. Anyone here have any thoughts before I take it in?

If it comes to that any suggestions who to take it to, or who not to take it to in the city?

First off to let you know I am a computer tech, own my own firm and have been doing computer stuff since 1992. Microsoft certified, Comptia A+, Cisco Cert, IBM Cert installer, and the list goes on. SO I know my stuff.

It could be a few different things: 1) Bad power supply no longer giving proper voltage levels and though it sounds like it is posting it isn't. 2) video card or onboard video chipset has failed 3) bad monitor (try a different one to eliminate this) 4) dead motherboard as a whole, biggest common thig to cause this is failed capacitors (the round tube thingys with metal top with X on it), have any of them bulged or split open

start with those and see if any help you. Not that I highly recommend them BUT the Best Buy Geek Squad is pretty universal in every city and though some of their techs are not that great they will get the job done (JUST MAKE VERY CLEAR, if they format your drive and lose all your data YOU WILL KILL SOMEONE, so make sure they know that is NOT AN OPTION.) they seem to do that too often and tell people it is your own fault when they lose your data.