i like to unfreeze my iMac 21"

my iMac 21" had a freeze problem and I had to take the computer to FairOaks Mall Apple store Genius Bar, but the computer did not show the free problem there. But as soon as I took the iMac home, the freeze probelm showed it immediately. I would like have your expert to diagnose the problem. Because the Imac is in frozen state, you cannot use screenshare , hence, we may have to use our iPhone or other way you choose.