Monitor suddenly developed vertical lines

This iMac had been working perfectly until I woke it up a few minutes ago.  It is a late 2013 iMac. What I got is shown below:

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In the next image you can see that the menu is still visible.

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I have restarted it several times but cannot login because I cannot see the login panes.  I have also removed the RAM and reinstalled them but that did not help.  Luckily my old 24″ iMac is still functional.

Any suggestions would be greatly appreciated.

Thanks!