I have a Mac Mini 2018 with two monitors. After upgrading to Big Sur the monitor plugged in with a USB C to HDMI converter was recognized, but the one plugged directly into the HDMI port wasn't. Turning the computer off completely and then restarting fixes the problem for me, but it's happened a couple of times now and is really annoying.
TheSteve
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution