Go back to basics as all old techs do. Combine and run the 5 things needs: PSU, motherboard, CPU, RAM, and video (if onboard use onboard).
* If it makes no beep code, usually CPU has an issue.
* If it doesn’t boot up, but beeps, remove the RAM, and boot it with NO RAM. If it gives you the correct RAM error code, CPU and mobo are working as expected. Also, if you are using more than 1 stick, test EACH stick in the same slot, and then test each stick in a different slot. Sometimes RAM gets borked, sometimes, the slots get borked (bad trace, damaged pins, etc). Repeat this step for each ram stick and slot. You want to basically boot each ram stick in each slot so that you know if there is a bad ram stick or slot.
* IF it boots up and asks for a drive, get into the BIOS and wait there for a bit (want to see if the CPU is overheating… sometime implies a very bad fan installation but not always). If it works after a few minutes, then attach a USB drive or HDD (your preference) and figure out if it’s an OS issue.
* If you go through all these steps, try changing the PSU. I have seen PSUs “working” but not providing full power on a rail or two (either in voltage or amperage).
* Make sure you are plugging into a reliable power outlet (sometimes I HAVE seen a bad outlet that has had some char marks on them and the user still used it, <facepalm>)
* If all is good, then you possibly have a bad mobo…