It’s the power supply. The power supply is like the liver of the computer. It has +12 volts, - 12 volts, + 5 volts, - 5 volts. Even 3 volts. Look at all those wires. 16. to the motherboard, 4 to each hard drive, and then there’s the wires to the video board. If any of them has a cold solder joint or a blown capacitor behind it, it won’t put power everywhere that it is needed. I’ve had power supply issues before. The computer fans turn on and some lights flash and it is really hard to imagine that the power supply is faulty. This is why I don’t build my own computers. I only buy used ones and only Dell. I have right now 4 dell desktop computers and 7 dell laptops. I bought a desktop brand new at Costco and the rest from Goodwill stores. I have had used and gotten rid of at least a dozen Dell computers that I bought previously owned from thrift stores.
Okay, count me curious. Why do you have all those computers?