NEED service password?

Why not just do a retune?

XXX
May 2016

TV sets are not all the same you know - and you do not mention the make and model.

Here's a link of how to make a factory reset for a Panasonic set.
http://eng-ca.faq.panasonic.com/app/answers/detail/a_id/20638/~/how-do-i-reset-my-tv-to-all-factory-default-settings%3F

If your TV works apart from a few missing channels - again you do not say which channels are missing - have you considered that they may be missing because of a weak signal from a very distant station.

Not all transmitting stations send out all the channels

Some Luck...
May 2016