MacBook Pro, Mid-2014, Dead screen I recently updated my mid-2014 MacBook Pro running High Sierra, which was working perfectly to Mojave. As soon as I did this my screen went pixelated (just like if you added noise on photoshop) and the display started to jump around. I turned it off and back on and it seemed to be fine. Around a week later while running Android Studios, it happened again but much worse. Turned it off again, however, was unable to turn it back on. 

The guys at the genius bar informed me that there is an issue with the display and they will need to replace the whole top of the Mac costing around $800 and take 10 working days. They told me that it works while being plugged into an external monitor, which it does but is incredibly slow.

Booting into safe mode doesn't help &amp; running the Apple Hardware Test doesn't report any issues.

How could updating to the latest OS have caused this to have happened or could it just be a coincidence? I haven't tried reformatting it yet but could it help? Anyone else experienced this or something similar?
How could updating to the latest OS have caused this to have happened
  or could it just be a coincidence? 


It's entirely a coincidence.  Many times, people incorrectly correlate an activity they are doing at the time with failure of an unrelated component.  You could have been doing a Time Machine backup, just the same, it it would have failed.


  I haven't tried reformatting it yet but could it help? 


You don't need to.  You can boot into Internet Recovery which is the same as booting into a "fresh" install of macOS.  You can also install macOS onto a flash disk and boot from there to test.  There's no need to reinstall over your existing data to see if you can fix your display.


  The guys at the genius bar informed me that there is an issue with the
  display and they will need to replace the whole top of the Mac


Yes, you definitely have an issue with your display.  However, I'm not convinced that it's your screen as it could be your GPU, backlight or the LVDS connector/cable.  I would take it in to a different, reputable shop for a second opinion.  See this video for additional information.