Gray progress bar appears under Apple logo during startup?

Startup Issues - Resolve

Startup Issues - Resolve (2)

Startup Issues - Resolve (3)

Sounds like it's trying to boot into Safe Mode, but fails because of directory corruption, which it can't repair. Boot to the install DVD and run Disk Utility>Repair Disk. Might need to do it a few times.

See this for more.

http://support.apple.com/kb/TS1417

The grey progress bar fills to about 20-25 % and then the iMac just switches off!

You have drive corruption and the fsck boot drive check can't repair it.

If you have a copy of your recent data off the machine,

Then option/alt key boot from the 10.6 disks, or command r boot into 10.7/10.8 RecoveryHD and security > zero erase (or moving slider one spot to the right) your MacintoshHD and restore from your TimeMachine backup or install OS X again and restore from TM or other backups.

If you need the entire drive erased, the use 10.6 disks to do that or hold command option r on a Ethernet connection to a router (or near your own) and use that Disk Utlitiy to erase the entire drive (moving slider one spot to the right), then reinstall OS X (whatever version) and then log in a AppStore upgrade, then restore from TimeMachine or other backups.

If you have a bootable clone with Carbon Copy Cloner or Superdupes, then merely hold option/alt and boot off that and grab files and reverse clone.

If you have no backups and need to grab your data off the non-booting internal drive, here are some options.

Create a data recovery/undelete external boot drive

My computer is not working, is my personal data lost?

If I'm not making any sense, then seek the services of a local PC/Mac tech that does data recovery and will restore your software, else any hardware repairs are better off left for Apple to do, especially if your under AppleCare or warranty as it's no cost to you, however Apple doesn't do data recovery.

I've tried all of the suggested with the same result: the bar loads for about an inch and then it shuts off.