Display Frozen. Please Advise

Hello,

The display on my iMac is frozen on a frame of a video I was watching using Chrome. The audio and other processes seem to be running normally though. Happened out of the blue and I can't use the mouse.

At the time the display froze: internet is hard to the ethernet port, I have one external hard drive connected via USB (currently backing up my HD to it), my keyboard connected via USB, a mouse connected via USB, and a small external display connected to one of the Thunderbolt ports using a VGA adaptor.

I figured in might be the external display (I've encoutered times in the past when the Mac was confused about it) so I unplugged it. This did not work of course.

Like I said, I can hear the video playing in the background (and I can see the external HD is still being fed files for the backup ). Is there anyway to just reset the display while logged in? I was hoping to avoid a hard reset with the backup still in progess.

Thanks in advance.

iMac 27" 3.4 GHz i7 EMC 2429