I installed Ubuntu 11.10 two days back. Today I added a user named vivi and set it to administrator. And I just changed vivek, my username, to standard user. Now, since I created new user vivi, I have not supplied any password for it, yet I am unable to perform any administrator actions. Can anybody help me to restore administrative privileges?
Not sure if the above answer will work, since under live cd, you can't edit the system which installed too easily.
boot from the live CD, or mount the drive some other way, then edit
/etc/group from your installed system. Make sure your have a line like
admin:x:119:vivi - don't change the number if it's different, just add your user name at the end of the line.
You also have the option of editing the passwd file, editing the root entry so you can login, but that's probably less likely to work well
If I understand yours problem, you have now no administrator account with password and just common user account (without admin privileges == not listed in suders (not in admin group).
Well - I thing yours only option is boot from live CD and (as root) set/change password for yours admin account (in chroot) or add his user account in admin group.