I have forgotten my root password after changing it. Is there a way to recover or reset it. I really do not want to reinstall Ubuntu again.
See my comments, but if you haven't done anything too drastic, this should fix your problem.
Press the super/windows key and launch the terminal.
When the Gnome Terminal opens, type the following:
$ sudo passwd root
sudowill ask you to authenticate with your password. This is your regular user password you use to log in.
Enter the new password you want to use for the root account, and press enter. The system will ask you to enter it again. Make sure you type it the same both times.
If all went well, you will see the message:
passwd: password updated successfully
As I said in my comments, the root account is typically disabled by default in Ubuntu.
I hope this helps you.