Problem scenario
You cannot make changes to an /etc/fstab file. It is read only. What should you do?
Possible Solution #1
If your /etc/fstab file is messed up (through manual changes to it), and your system boots it as read only, can you run a command like this?
sudo mount -o remount,rw /dev/mapper/foobar /directory/to/be/mountedIf you run "sudo cat /etc/fstab" you will see lines like this:
/dev/mapper/foobar / ext4 defaults 1 1(If you saw the above, and that was the line that was corrupt, replace "/directory/to/be/mounted" with "/".)
If it doesn't work, with modern Linux OSes and modern hardware, you can boot to rescue mode without installation media (e.g., a CD, DVD or USB stick).
Possible Solution #2
Use "sudo". Sometimes when you are frantic, you forget that normal users cannot make changes to sensitive files. Can you assume the root user?
Possible Solution #3
See one of these three links:
https://www.unix.com/linux/54874-how-edit-etc-fstab-when-root-mounted-read-only.html
https://linuxtect.com/read-only-file-system-error-and-solutions/
https://forums.fedoraforum.org/archive/index.php/t-144176.html
There was a good stackexchange.com (e.g., unix.stackexchange.com) posting and answer for something similar that involved booting up, then moving the file system temporarily to RAM and/or /tmp/. If you know which one this was, please post it in the comments below.