My 1998 GMC TOPKICK 7500 head lights quit working but all other lights work what is the problem?

Its should be one of about 3 answers.
1) Under the hood of your truck inside a black plastic box, is a set of fuses and circuit breakers for:
a) electrical items found in the front of the truck, and
b) electrical items which take a large amount of electricity.
Look at the fuses and circuit breakers in there for a problem.

2) You have a switch that turns on the headlights.
It can fail ( but rarely does). It is your dashboard switch.

3) You may have a "high beam/low beam" switch that you activate (like a turn signal switch) to change high to low or visa versa. It can fail.
God bless your efforts.