I created a new user and set its option to "root". Then I switched the option of the user which was activated to "standard" and closed the window. But now I can't login as the new user because I didn't set a password and so the user which is root is not activated. What shall I do?
Does GRUB show you the recovery mode? If yes, select it, and then
If you don't see GRUB, hit Esc or Shift (depends on your system)
If even this doesn't work, boot a Ubuntu Live System and do the following things: