Dell optiplex 7020 no display

Goodday, I have 2 dell 7020s that I'm using at home. The 1 had a power supply and so i was using the one which was functioning all right. I recently got a replacement PSU for the faulty one and after installing the PSU i took ram out od the one which was working for testing and all went well. I re-installed the ram back in the already working PC but I can't get display from it.

The power led stays solid white (no blinks), fans and everything else seems to be working fine but there is no display. If I start the machine with all ram modules removed i get the beeps to signal no ram installed. I've tried to swap the ram in the different slots and individully (i have 3 * 4gb sticks) but no luck. If I put the ram in the other Pc its working normal.

I've even tried swapping the PSU, the Processor, tried running with nothing connected on the mobo but i still get same respond. 

Start PC with no ram- i get beeping codes for ram

Install ram one by one- no beeping sounds, no display.

Ram works in the other PC no issues, monitor works on the other PC no issues..

Any ideas what i maybe missing, need this machine to be up and running again.

I'm using onboard vga port, no added cards on.

Thanks, I hope its enough infor..