How come a light fixture wont turn on after a grease fire? when it was dont burned.

The fact that a light switch doesn't work isn't necessarily saying the problem is inside the switch. It could be the line that feeds the voltage to the switch is open somewhere in the circuit. It could also be a coincidence. I'd trouble shoot it the same as any other switch problem.

With the switch on, measuring to ground:

If there is voltage on both sides of the switch (to ground), the problem is between the switch and the light fixture including the fixture.

If there's voltage on only one side, the switch is bad.

If there's no voltage on either side, the problem is between the switch and the power panel. Check the fuse first.