iMac won't restart.

The past days have been really hot and our Computer was down for awhile. After this, our iMac has been slow (Mainly Safari, downloaded games were fine) and the beach ball continuously came up. I tried to clean it up a bit, it didn't work. I tried shutting off the Computer and turning it back on, nothing. Finally I restarted the Computer and it finished but I went to Log in and the beach ball shows up! I tried to hold the Power button and hold 'D' to restart it again. A loud beep occurs, it starts to restart but shuts down a quarter way through. Completely off. Basically, our iMac shuts down in the middle of restarting.

Please help! We don't want to bring it into a Repair shop. :(