My headphone is not working one side

Hello,

See these steps:

1) Check the charging port for lint or other debris

2) See this: If your headphones don‘t work with your iPhone, iPad, or iPod touch

3) Warranty: Apple One (1) Year Limited Warranty

4) If you think that the headphones are defective, have them checked at a store when they open back up: Find Locations

Good luck.