Dennis' 2011 iMac is showing lines on a white screen. Leo says it could be a failed monitor, or even the cable that connects to your monitor has failed. The OS may have been corrupted as well. Try booting into recovery mode. CMD+R. That will bypass the loading of the OS and will allow you to reinstall the OS. You can also try target display mode. If the problem persists, the Mac needs replacing., because it'll cost too much to fix.