Blank Screen on iMac

19.5 in 2011 iMac running Yosemite 10.10.2.
Came down to a blank screen on my iMac. Usually touch a key and it wakes up. Now what I get is just a blank screen, dark but not off. The curser works and I get the beep sounds when I hit keys on the key board.
Tried a hard shut down a couple of times where the Apple sound plays and the apple appears and it shows as loading, about ¼ of the way loading, it stops and reverts to the blank screen. Any ideas as to fix?

Dear Diver I hope you did not receive my reply prior to this edit. One of the drawbacks of using a portable device is no option to edit. Somehow I deleted the first paragraph of my reply. I apologise if I gave you the wrong impression of the situation.

So the first thing to do is try to start up in "Safe" mode by restarting while holding the Shift key. This eliminates the possibility of a program causing your problem. If is starts up that is a good sign but either way the next thing to do is try starting up from the recovery partition. Shut down and restart holding the Command and "R" key. Hopefully you will boot to a window with 4 options. Select Disk Utility, select your Macintosh HD (the second one in the left column)
and click Repair DisK or if you wish to be careful click Verify Disk first. Hopefully you have a complete backup of your HD. If the HD is repaired successfully, quit Disk Utility and restart. If the HD is OK you may want to Repair Permissions as well while you have the chance.

If you cannot get to the recovery partition at all you have a serious failure.

The bottom line here is, discounting the possibility that this is hardware problem for now, you should try to boot the iMac from another source other than the internal HD. Preferably this source should have the current OS Eg. A bootable clone of your HD or OSX installer on a separate drive.

If you have no success with any of this then you can assume it is a hardware problem. Either your HD has failed or there is a problem with the internal circuitry.

Thank you! Thank you! Thank you!
Safe start up did not work but did from the recovery partition! Had full Time Machine back up on Time Capsule. Thank you again!

Hi Diver forgot to say welcome to these forums. So does that mean a successful repair of your HD? If so have you noticed an increase in the speed of operation now?