You really should find out what happened before you start sending stuff back. The green light on the mobo isn't really telling you much. Go buy a PSU tester, they're cheap, or swap the PSU and see if it works. My guess if you had a 20+4 connector for your motherboard and an aux 12v; then plugged the +4 into to aux 12v.
If you have a square 4-pin connector on the board, usually the wires going to it will be yellow an black (although, not always check your PSU documentation).
Anyways, check the PSU with a known good or with a tester. Be sure to unplug everything except the PSU, CPU, RAM, and video card if you don't have onboard video. And see if it will boot. Any POST errors?
If it doesn't boot with just those and a known good PSU then you probably fried your mobo. Replace it, be sure to use stand offs and KNOW WHAT YOU'RE CONNECTING BEFORE YOU CONNECT IT.
If you're still having problems after that, you probably fried something else. Swap RAM, CPU and video card if you don't have on board video individually until you find the problem(s).