ELECTRICAL PROBLEMS?@!?

I am having a little bit of electricity problems, I have a room in my home that is now an entertainment room, my problem is about 2 weeks ago the electric stopped working in this room, it is controlled by breaker 15. One receptacle is tied off and has a cover on it, it has 2 seperate 12/3G's coming out as service wires I'm assuming the reason for this is one is maybe for the ceiling fan, & the other is the receptacle, the other has a GFCI outlet hooked up to it with a 12/3 G service wire to it but when I hooked up the GFCI tied off the 2 12/3's again made sure everything looked good it threw the breaker, so by throwing the breaker you know that is telling you, you atleast have a good current going out to what it is servicing you just don't have something aligned just right. So if someone could help in anyway please do I need to get this room up and going