Whenever something is not working, there is a fault finding method that can be used. It is "replace one piece at a time and see what happens". So, you have a servo that is not working.
The #1 check should be: Connect a different servo up, see if that works. Preferably one that you have tested elsewhere and know it works.
What other pieces are there? A harness/wire?
#2 test; change out that
#3 test; change out the receiver
#4 test; change out the transmitter
#5 test; change out the battery
Never assume anything is working properly;
Never assume anything could not be causing the problem;
But start with the most likely suspects.
This works for a wide range of things, not just model airplanes