Diamond HD4850 1GB GDDR3

I just built a new computer from scratch with new pieces (including monitor) yesterday night and left it there until this morning. However, the first time I ever booted it up, nothing showed up on the monitor. The monitor didn't pick the computer up. At first, I thought the monitor was broken, so I switched it to a reliable one I've had for a year. Again, same results. Then I changed the DSUB cable, same results. I opened the case and I took out the card and I put it back in, same results. However, throughout all of those tests, I've noticed one thing: on the graphics card (where I assume the problem is) there's a little red light that flashes up and above it in tiny print says overtemp. By the way, yesterday, I left all the pieces in the car, and they did get quite hot. Anyhow, I was just wondering, if anyone could post a clear answer, or better yet, a tutorial on how to fix this. If I still can't, I can still bring it back to the store for a refund, but I'd like to test my new computer first. The graphics card is a Diamond HD4850 GDDR3 1GB
Thanks in advance.

hey everyone, its the asker. the eight pin CPU power connector was loose, thats all. thanks though!