Loading gif and progress bar on start up After doing a pram and a smc reset my late 2011 mac book pro 10.8 shows the apple logo on a grey screen with a loading gif and a progress bar which once finished the mac just turns off. 

I have got the error in the disk utility of my hard drive. "Invalid index key"

I am currently backing up the hard drive in the disk utility!
It seems your hard disk is failing. Backup all your date with Time Machine. Then boot in to Recovery Parition by starting up and holdind the keys cmd-R. Try to repair the disk with Disk Utility. Anyway, consider to replace it.