Which fan did the shop replace? was it the CPU cooling fan?. Do you get the a single "BEEP" after POST? if yes, try starting in safe mode (just keep pressing F8)from here you should be able to trouble shoot.if NO,I would go back to basics, disconnect everything, except the GFX, the pc should boot and you should get a display showing text only.if no display,it could be your MB thats at fault(i have ruled out the CPU, as they either work or dont,they dont go intermittent)
You should also contact ASROCK, could be a problem with the MB. Also check for a BIOS upgrade.
see how you get on