Sometimes you cannot edit and save a single file in Linux. For example, if you boot into maintenance mode after the /etc/fstab is corrupt, you cannot edit the very file that is causing the problem. To overcome this problem, you will need to log in as root and issue a command. You will need to manually edit the /etc/fstab. To do this in maintenance mode you need to issue this command from the prompt:
mount -o remount,rw /
After this, you will be able to edit the /etc/fstab file. Hopefully you will know what is wrong or you have a backup. Be careful when running scripts that automatically update this file. If the scripts haven't been tested or cannot handle exceptions, the /etc/fstab can be modified in unusual ways.