It sounds like a logic board issue but you also stated you touched the thermal connector and its disconnected...if that's the case you'll definitely get a Veep code and no boot as the Mac us essentially protecting itself. Make sure EVERYTHING is reconnected. RAM will always produce a 3 beep pause, 3 beep pause over and over so a single Veep,is not the RAM. Make sure the data connector to the logic board didn't pull out when you swapped the drive, did any fan connectors come loose? The newer Mac minis have a fan connector that can be ripped off the logic board very easily if you lift the assembly up to high before removing it, it's very, very touchy and easy to come unsoldered from the logic board by a simple wrong touch. I pulled mine out very, very, very carefully and it still came. Loose took the receiver clip off the logic board with it when I tried to unplug it, which is a horrible design, I'm not sure if the older ones have this or not, but you have to make sure every single cable is re plugged in or you're going to get beep codes and not boot up. So I would reverse the process and check everything very carefully and be very patient as to not rip a cable. Or have something come loose from the logic board? Otherwise you could very well. Need a new logic board as some of the stuff is so small it can't even be soldered but if you're getting a single beep code you definitely have a logic board issue or something connected to it that isn't and so it's producing a code