The most likely issue is, as you assumed, your power supply. Not exactly in the sense that the power supply is broken, but you will want to make sure all of your cords are in the proper spots and that they're all nicely snug into their ports, otherwise your pc won't turn on for safety reasons. It's not likely that your psu or motherboard are dead. Try making sure you plugged everything into the right spot on the psu and maybe even try switching around some of where things are turned on. Also, you don't need everything to turn on immediately to make sure everything is working. Here's a video to check your psu alone to make sure it's working so you can hopefully eliminate a culprit to what may not be working in your pc...