If you’ve accidentally screwed your /etc/fstab file and rebooted, you might run into some issues while trying to make changes to fix it, i.e.
Error writing fstab: Read-only file system
Fixing Error writing fstab: Read-only file system
In order to fix this issue, you will need to remount the affected mount-point with read / write options.
This can be done by issuing the mount command:
mount -o remount,rw /
Where / indicates your root mount point, change accordingly if you have borked a different mount point.