MacOS Sierra bricked my iMac

ok, so both of those things have now been done. The computer itself is clearly booting up, because this time I could see the cursor was blinking for me to put my password in, which I did and pressed enter. Then I could tell from the colours and flashing that it was loading up my desktop. However it still looks like this so I can't actually do anything or see what to do?

literally sat here unable to work, so Frustrating!

any further ideas anyone?

good idea! i had a second monitor connected and it wouldnt load to start with so I disconnected it in case that was causing the problem. Now I've just connected it up again after I've managed to log in and that displays fine, with my iMac screen still having lines all over.

SO should I try installing Sierra again? Can I roll back to El Capitan? What's the best way forward?

OK, so I eventually managed to re-download Sierra and reinstall it. Sat here watching to see if there were any clues, but nothing out of the ordinary happened, it installed, restarted and then the lines just didn't go away.

This really is so frustrating. Next step I guess is to re install El Capitan?

but what I don't understand is why this has happened? I have a 2014 iMac, no modifications, always runs fine. I've installed other updates with no problem. But i can't find anyone else having the same problem so it's not a common thing?

It boots fine, I've got a second monitor connected now and set to mirror so I can use the computer aspect of it on the second mon and the OS itself is working fine, but the display on the imac screen itself just permanantley has these horizontal bars running up and down so I can't see anything on the screen, so presumably it's some sort of driver issue?

JJust trying this, using apple diagnostics, but it would appear that it uses that before it loads the second monitor, so i can't see anything on the screen on my iMac!

Im not sure how that's going to work?? The whole problem I've got here is that my imac display isn't displaying anything other than flashing lines as per my screenshots above and this whole post??

OK just done that and no change. Lines up and down the screen and cant see what it says