did it ever boot up at all? Its probably the logic board if the PMU doesn't reset. If the mac mini is under a year old then its still under warranty and you can get it fixed for free, new logic board and everything. Just take it to an apple store and tell them its still under applecare for being under a year old.
Well the same thing happened to me, I opened up my mac mini tried to troubleshoot it then took out anything I had put inside it and brought it to the apple store and asked them to repair it under the applecare plan. It was free. There's no sticker or anything that you break by opening it. I'd say just take it to an apple store and try it. I bet they don't even ask about whether you've opened it or not.