I’ve had this happen a couple times. I’ve had some success resetting it… totally removing it from ST… (a pain since I had to remove it from all my smartapps), then pulling battery, and putting battery back in while holding that button down, re-pairing it, etc. Sometimes I’ve had to do that 5-10 times before it works (and I usually try different combinations of order doing the steps… pull battery, put ST in pairing mode, put battery back in… or hold button and put battery back in, etc…).
Then sometimes that doesn’t even work. Is it a SmartThings sensor? If so and you can’t get it to work, you can contact customer service. They will usually try a few things on their end and if that still doesn’;t work, they will often replace for you.