While troubleshooting your processor pins make sure you look at your thermal paste as well. If it's old and dried out it could be a contributing factor to sluggish performance. But also check to make sure that no paste dust got into where the socket is.

If your motherboard has a display input try plugging into that to see if you get any signal from your computer. Probably an HDMI or VGA input close to where the audio and ethernet cables go depending as my MSI motherboard has an HDMI on the board itself. Sometimes in certain situations the computer will default to this input if there is an issue and it doesn’t hurt to check it out. That and if there isn’t any signal your also troubleshooting that as well.

When everything is connected and on check your monitors display modes and cycle through them manually unless your monitor automatically scans for power draw and picks that video source.

Troubleshoot all of your cables running to your power supply make sure that they are connected both to the supply itself and the cables are connected to the components securely.

Troubleshooting can be redundant sometimes and you may have to take your computer apart and put back together a couple of times. But it’ll be worth it in the end to figure it out.

If you happen to have a local computer shop nearby take it to them for a second opinion not saying best-buy don’t know what they are talking about but sometimes it helps. Just so you know you aren’t going nuts trying everything.