First try reset CMOs jumper and replace CMOs battery.  If that doesn’t help test motherboard with a different verified working psu.  Computer shop may not have done thorough diagnostic test of your pc.  These days not worth investment to replace old motherboard in this model.  Lynnfield Cpu in 8100 is quite old too.  You can install a newer gen motherboard cpu combo in the case if the current board is indeed bad.
10 years old. Bought new as a back-up and has less than 10 hrs of running time. When turned on last week, the start button amber light flickered on and off. I shut down the PC and tried starting it again. No light. Took it to a computer shop and was advised that it needed a new motherboard. Recommended that I buy a new PC. The motherboard number is DH57M01. eBay has a few with that number. Will any DH57M01 work in my PC?
Studio XPS 8100, motherboard