It’s very odd that the shop would not be able to troubleshoot this within an hour. The standard procedure for your error is to unplug everything and reseat it with minimum plugged back in. You’d test for CPU, PSU being plugged in correctly (24 pin especially), RAM (one stick at a time in different slots), and finally the board itself.

Literally should take 30 mins or so to do this if you have a working spare computer.

My bet would be on memory issues - those are usually most common reasons for not POSTing but could be video too. Motherboard and PSU are rarely the issue - especially if new. Memory often comes faulty even brand new.