There are a number of reasons why a hand warmer may not be working. Air activated warmers have expiration dates, so if that is the style you are using make sure that it hasn’t gone bad. With all reusable warmers, make sure that it has battery or fuel before deciding that the product is broken.
A common reason that it might seem like your hand warmer is broken is using it in an unconfined space. Hand warmers are designed to keep small spaces of your person warm. Their heat output is high enough to warm a pocket or glove, but won’t be very effective if you are trying to use it in the open air. If this is how you’re using your hand warmer and it doesn’t seem to be working, place it in a pocket before giving up.
If your hand warmer does break, reach out to the company you makes it and see if they have any advice or are willing to help you get a new product. Most of the time they will want to ensure that you have a quality customer experience.