No problem
I work with a lot of Dell machines so I am used to troubleshooting them
I think we may have crossed some wires here by accident - when I said the power button, I meant on the base unit - not the screen! lol
Make sure the CPU socket is nice and clean (no dust and bits in there) and the little gold connectors on the bottom of the CPU have no smudges on. You can use a clean cloth to wipe the underneath of the CPU, as sometimes after handling it you can accidentally get thermal paste on the connectors. It's the CPU SOCKET you have to be delicate with, as there are tiny pins that connect to all of the points on the CPU itself.
If you are happy with this and have tightened up the heatsink as much as you possibly can, here are some more things to try if you are still having issues -
-Re-seat all of your memory modules, or try just one module at a time. You may be able to isolate a faulty one.
-Take out your GPU and try running with just the onboard graphics port.
-Remove your CMOS battery for 30 seconds
-See if you can locate a "Service Mode" jumper setting on the mothboard somewhere. (Not sure if the 720s actually have one but worth looking)
-Remove any other add-on cards you have installed. (Wifi cards, NICs etc...)
Good luck