Only one side of my headphone works

Hello,

This is the Apple wired headphones, correct?

If so, check the charging port for lint or other debris and see this:

If your headphones don‘t work with your iPhone, iPad, or iPod touch

Try cleaning it: How to clean your AirPods, AirPods Pro, AirPods Max, and EarPods

If no help, check the balance:

Go to Settings > Accessibility > Audio/Visual and scroll down to “Balance L vs R” (Adjust the audio volume balance) and make changes.

Good luck.