from enable mode # issue the command erase startup-config, then reset 838 and don't save the config. It will come up with factory default, you will use ADMIN and PASSWORD to get back into the 838
On another note, if you can not log into the 838 you need to call Adtran support, you will need the serial number of the 838. Instead of using ADMIN, you will use challenge for the username. It will give you a code which you give to support, support will give you a code to enter back in and it will allow access, at that time follow first instructions. Side notes, the person who first answers the phone can give you the challenge code, just let them know that is what you are calling for. Also the challenge will time out so enter it while you have Adtran on the phone.