I had that happen a few months ago. the wires inside the headphones had detached at one of the solder points. I just re soldered it and put a zip tie on the inside part so the wire couldn't get pulled again. Inside they are actually a really cheap setup with flimsy wiring, I'm surprised mine lasted as long as they did.
My ear phones did the exact same thing shortly after I bought the Pro. The stock head phones are garbage. I bought the water proof ones to replace them. After 8 yrs of use they still work like new. Great headphones, no volume control which I never felt I needed. They are durable and will last you many yrs.