I cleaned my computer inside/out after I removed the PSU, fans, front panel, and SATA cables. When I put it back together, I pushed the power button and the GPU fan instantly went into high RPM with no display to show for it. After holding down the power button for over 30 seconds, my computer refused to shut down.
A local computer technician spent a weekend looking at it with no success. She suggested I replace the mobo & cpu to see if that solved the problem.
After doing so, same problem. The only difference is that the computer shuts down INSTANTLY when I push the power.
Not to undermine your intelligence, but are you sure you plugged everything in correctly? It might be your power supply that crapped out. I dunno why she told you to change the cpu since you didnt remove it and those rarely fail.
When I replaced the motherboard, I made sure that all of the Front Panel Connectors were in the right place. I have 1 PCI Express connected to my GPU because that's all it has. Both of my SATA cables are connected to the HDD & Optical Drive. Both are powered to the PSU. I wish that was the case. My fans have their own designated string connected to the PSU. My PSU is modular & 700W. More than enough power.