read-only root file system
How to fix root file system is read-only problem

If you have problems in linux it can go in error mode when root file system is mounted read-only, that prevents you to make changes to system settings and therefore to fix problems.
It can report that root file system is read-only or that root file system is not mounted
In this case you at first must set root file system back in read/write (rw) mode.

To do so you must know on which partition is placed root file system that you can check using fdisk command and then option l (lollercase L) to list partitions

Then remount root file system back in full access mode with command like this:

mount -o remount,rw /dev/sda3 /

where
/dev/sda3 must be replaced with device path where is placed root file system in your case
/ is path of mount point of root file system
rw specify read and write (full access) mode

After that you can make configuration changes you need