The G5 was having post issues the other day. She would spin up the rear fan, GPU fan, and light up the RGB on my CPU cooler, but no video or CPU activity. 
 

Made sure all the connections were solid and flipped the PSU switch off and on, seems to be doing fine now. 

 

The PSU in there had come over from my previous build. It’s like 5 years old. I thought it could have been dying, but I’ve been using it since and haven’t seen anything out of the ordinary. 
 

The wall outlet that this thing runs off of also has a pretty power hungry AC running off if it and a large power strip powering my monitors, dock, 55w speakers, and radio. It’s possible that the combination of my non-standard power button and the AC being on means that the PSU didn’t kick on all the way.