There are three possible causes.
1. Bad speaker
2. Bad cable
3. Bad main unit.
Connect the center speaker instead of one of the front speakers with the front speaker cable.
If it works then try it with the center speaker cable connected to the front speaker output.
If not then the speaker itself is bad.
Hope it's just the cable.
If it works with both the center speaker and cable connected to the front speaker connection then the problem is the center channel amp in the main unit. That's the worst case scenario. Usually not cost effective fixing them.