The linux kernel remounted the filesystem in read-only mode. This usually happens when an error occurs when it tries to write something to disk.
If you have a backup of your system (stored on a different computer) then you can try to give it a reboot. If you don’t have a backup, then try copying the files you would like to keep to another system before doing the reboot because there is always a chance that it won’t boot up anymore depending on how severe the issue is.
