Was it once under some complicated circumstances or you saw it once again after you made the restart?
May be to save all your data before to try again (if you did not) and on some free time have a look into [HowTo] reboot / turn off your frozen computer: REISUB/REISUO
Also, what journalctl -p 3 reports on that first time you saw it (if it was the previous boot up, then add -b -1 argument)?