Just follow the below steps to recover it.
1. Reboot your system and at the GRUB press “e”. Edit the first line and make sure it says “linux single”. Then press “b” to boot the new entry line.
2. You will now have access to a BASH prompt. Enter the following commands:
and look for a line somewhere at the top of the file that looks like this:
You must remove the “x” from that line so it will look like this:
And save the file.
3. Run the following command:
Edit the first line in the shadow file by removing the encrypted password. The line must now look exactly like this:
Now save the file.
4. Reboot the system and log in as a normal user, then set your new root password using the following commands:
$ sudo su