If some administrative program like Users and Groups does not work, reset the default permissions

sudo usermod -aG adm,sudo username

If sudo does not work, try

su
usermod -aG adm,sudo username

If su does not work, boot to the init-shell. For the grub line add

rw init=/bin/bash

… and on the command prompt
usermod -aG adm,sudo username

After permission resetting log out or reboot.