I had this same error a few months ago when a drive was failing to mount because (unbeknownst to me) it was overheating. I turned the system off for a few hours before troubleshooting (I had errands to run). When I came back, the system booted without issue.

If you’re looking for an easy troubleshooting step, have you tried turning off the system for several minutes (and unplugging it) and then powering it back on?