White screen, but still responsive

My iMac has a solid white screen (no folder or "?" icon), but still seems to be responsive.

The computer still responds to the volume buttons on the keyboard, and makes the corresponding sounds. It also responds to the keyboard's brightness buttons. It also seems to still respond to the mouse, as it makes the muffled "boink" sound when the cursor (presumably) goes to a corner where I don't have a cascading window function mapped.

It doesn't seem to be the white screen of death I've read about, and seems to start up and load the OS. I've done the P,R, Option, Command restart to no avail. I didn't witness the screen going white, so I can't say if it was sudden or not.

Any ideas?