In some cases (kind of maintenance mode for instance), partitions can be read-only mounted, and the situation stays the same after operations (like fsck) and reboot.
For instance, it can happen if the system is wrongly configured, forcing the fsch of unknown (or removed) devices.
To leave such end-less situation, it is possible to remount a read-only mounted partition, which can not be umounted (like /), to update configuration and then get a working boot:
mount -n -o remount,rw /mountPoint
The -n option allows to request the mount without attempting to write on the disk (which is obviously not possible on read-only mounted partition).
The -o option allows updating mount options (same syntax than when mounting the partition the "first" time)