MacBook Air 2011 stuck on update. Receiving “cannot update MacOS”

I tried downloading High Sierra from El Capitan and my computer had to shut down and restart and download and install the update. Well now I keep getting a message saying it cannot update MacOS and the only option I have is to restart my laptop and it's just in a constant loop. It will not update and now I can't access my computer at all. I can't get past the failed update. Even with a hard restart it does the same thing. Idk what to do. My laptop seems absolutely pointless now and it's worked perfectly for years