Hard disk gone

So i had left my mac on and went away and did something else for long enough that my went to sleep, when i came to it again i tried to get into the computer to get on it again, how ever the screen went black with the mouse pointer. The mac stayed like this for ages (5-10 minutes) so i decided to hold the 'on' button so i could manually restart it, after turning it back on again it went onto the disk untility and didnt actually say it, but the Hard disk was gone, i have backups from timemachine and another external hard drive but cant boot up from the hard disk. Just comes up with 'disk 2' and that disk is ok but i dont know what it is. I have tried turning off and back on again of course and nothing has changed apart from i have gotten a little more confused. The mac did feel very hot, but couldnt smell burning of wire or plastic or anything, also i hadnt moved it at all when the innocent happened or before.

Help much appreciated.

cheers

Robin

P.S. Apple mac mid 2007 Running: (very slow) OS X.Latest from today (1st september 2015)