Help please

First turn off your IMac and turn it back on then do the following.

OS X Recovery includes a built in set of utilities as part of the Recovery System. You can use OS X Recovery to do the following:

• Restore your Mac from a Time Machine backup.
• Verify and repair connected drives using Disk Utility.
• Check your Internet connection or get help online using Safari.
• Install or reinstall OS X.

To start your computer from Recovery, restart your Mac and hold down the Command and R keys at startup.

Hold these keys until the Apple logo appears. After your computer finishes starting up, you should see a desktop with an OS X menu bar and an OS X Utilities window with the options listed above. If you see a login window or your own desktop instead of the Utilities window, it's possible that you didn't press Command-R early enough. Restart your computer and try again.

Sounds like the OS is corrupted so I would use Disk Utility.

1. When your computer finishes starting up, choose Disk Utility from the Utilities window, or from the Installer menu if you're started from an installation disc.
2. Click the First Aid tab.
3. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
4. Select your Startup Disk (usually named "Macintosh HD").
5. Click Repair. Disk Utility checks and repairs the disk.

If that does not work then I would recommend re-installing the OS from the recovery prompt.

Your HD appears to have failed and is need of replacement. If the machine is still covered by AppleCare contact them to arrange replacement. If it is not covered, take the machine to your local Apple Store or AASP for service. If you do not know of any AASPs in your area please click https://locate.apple.com/country to begin locating one.

Good luck.