You can try resetting the pair.
Press the button on both for 15 seconds. Then power off. Then restart / re-pair as per normal.
Thanks for the instructions on this, I did this and paired afresh and they worked fine again. I think @Slater is onto something about them dying on me at the gym probably because they're not, as they claim, sweat proof as advertised, so I'll probably avoid using them at the gym ever again. But glad they at least work again! I wouldn't have likely even have bothered without your instructions on resetting.