My iMac screen is 1/4 white and 3/4 black with a bar code rainbow across the middle?
My wife was on the internet surfing and all of a sudden her screen did the above?
iMac, iOS 7.0.4
Restart the computer, if the problem persists run Apple's Hardware Test http://support.apple.com/kb/ht1509 , run it in extended moded at least 2-3 times back to back, if errors appear this confirms a hardware problem which means it need to be taken in for repair. If the machine is covered by AppleCare this will be covered, simply give AppleCare a call.
BTW without a completed profile it's very difficult to help. At the very least please let us know the version of OS X installed.
Good luck.