Syd has a 5 year old iMac, and her husband recently knocked it off the desk, cracking the screen. She's been using it still, but the crack continues to grow. Apple says it'll cost almost as much as a new iMac to fix it. Can she use another monitor? Leo says yes. You can consider your iMac monitor dead and use Thunderbolt 2 or HDMI to connect a second monitor to it. But you'll also have to go into the display settings and select "mirrored" to put the video signal onto the new monitor.
Another option is to replace the screen yourself. IFixIt has videos and tools on how to fix it, and it will only cost you in parts. Or you can call a local, Apple Authorized computer dealer to do it, and save a lot more than what Apple will charge.