You need the Private Label remote control in order to access the
service mode to change it back.

PL = private label, brand name the same Zenith tv was sold under that
was not Zenith.

If you have a good universal remote control with a menu button that
will work a Zenith, it might have the private label code in it. You
might need to do a lot of code searches to find it. I think Montgomery
Wards was one private label brand , as well as several others.