I've got an HP Envy 750se that is about 5 years old now, and I wanted to upgrade it. I bought a new case and new PSU, and got a friend's old GPU. After putting things together (and with a lot of troubleshooting), I was finally able to boot up the PC with the new components plugged in. I updated my graphics drivers, then shut everything down and unplugged some cables to start some cable management. I've since not been able to boot the PC and get any display output.
When I do press the power button, the CPU fans, tower fans, and GPU fans all turn on. I get 3 long beeps and 2 short beeps, According to this guide, this is a memory initialization error. I've tried reseating the RAM several times, in different numbers and different orientations, but I can't get the beeping to stop. I'm not convinced the RAM is bad, since I got things working prior to this latest issue. I've checked each cable connection, and nothing seems to be incorrect or loose. I've also tried using the old PSU, old GPU, and no GPU - still beeping.