HP computer won't boot from sleep mode and now doesn't even boot

HI,

I have a HP all in one computer running windows 7, about 4 months ago it stopped working. It doesn't wake from sleep mode and now doesn't even boot.

I completely turned off the computer and that's when I would only hear the fan running.

You can hear the fan running but nothing happens and the screen and it stays black (not sure how to force it starting into BIOS)

I removed the HD and installed an one thinking it would boot but nothing happens, I have a Harddrive hub and can't remember if the drive even spins. Nothing popped up under my computer when using the drive hub

my only other guess would be a power supply issue.

I"ve never had a HD completely die and we don't even hear a clicking sound.

I"m just very lost on what steps to take.

Thanks for the help