That tells me the file system on the disk is likely damaged and the system chose to mount it read only instead of the normal read/write.
I would boot to a live install media then run fsck on the system disk partitions, following which they could be mounted to verify they now have a clean file system before rebooting to the normal system.
If the problem still happens following a reboot, might be best to check logs for first signs of complaint, what happens right before the file system is flipped read only.