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.
target display mode
Mark has a 2012 iMac and a 2018 Mac Mini, along with a 2016 MacBook Pro and 12" iPad Pro. Can he hook up his Macbook Pro or his MacMini to his iMac and use the display? It can on a limited number of iMacs and is called Target Display Mode. IT requires a thunderbolt connection. Here's how - https://support.apple.com/guide/mac-help/use-an-imac-as-a-display-mh30822/mac. It may not work with Mark's model, but you can do it. Only for models mid 2011 to late 2014.