Why does one side doesn't work anymore while the other one is fine?

It's not the newest kind of headphones but i liked them a long time. My problem is ... Actually the battery on the right side but in general it works perfectly. The left side worked a long time then stopped Partially to work and finally since a few weeks it doesn't work at all... I'm a little bit lost and hope to hear that there is a way to fix it instead of buy new ones...