I was prompted to update to Big Sur today and am now unable to get past the login screen. Unfortunately I wasn't paying great attention to the installation but I did notice at one point it seemed to have failed. I was close to not having enough space on my hard drive which it seems might be related. However, Big Sur refused to install until I removed some items and then it appeared to have enough space. Currently, it can bootup to the login screen. I try to enter my password and it appears to be loading an update for a moment, but then it kicks out to what appears to be more of an installation screen for a moment, then eventually it just restarts and brings me right back to the login screen. Have tried most of the solutions below, nothing is working. I currently do not have easy access to another Mac such that I can load this externally. I would threaten to stop buying apple products, but I still think this OS (for me) blows away anything else I've ever used.