Not at boot? You mean it becomes read only after being read write all day?
The filesystem going read only means it became confused and is stopping writes to hopefully avoid corrupting the filesystem.
If the revert to read only happens well after boot, rather than failure to remount read write during boot, login remotely and monitor with 'journalctl -f' chances are that remote machine will capture the central events just as it goes read only and before whatever else kills sshd preventing you from remotely logging in after the fact.
Chris Murphy