How can I regain admin privileges?

Solution 1:

Following up on the comments above:

Because the admin group doesn't exist on your system we are assuming you are running 12.04. To be able to administer your system with 12.04 you need your user-id to be in the sudo group. I was able to add a user-id to the sudo group as follows.

Reboot using grub and select an entry for recovery mode.

Select the "root" option from the Recovery Menu. From the terminal that opens at the very bottom type:

mount -o remount,rw /

sudo addgroup your-username sudo

Then you can resume the boot by holding control and pressing "d" and select resume from the Recovery Menu.

As explanation, in recovery mode you come up with the root filesystem in read-only "error mode". The first command makes the file system writeable. The second command adds your-username to the sudo group.

When you continue the boot, or somehow reboot, your user-id will be able to use the sudo command as normal. Once you have sudo privileges you can add another administrative user and see what groups they are in, and then you can add your account to those groups, too.