Well, for 1 that can't happen as you would be warned about doing something like that and if you did you are left with one option.
Re-installing the os and providing the license key provided with your os disk!
ps. in the future don't do stuff like that!!!
Been there done that .... not myself but have had users add a user and once done, the original admin account is not visible. A search of the web should bet you numerous options: