An API key may stop working in those occasions:
  1. It was manually revoked by the key's owner;
  2. The API access was turned off by a team administrator;
  3. The password of the account it was linked to was changed;
For security reasons, changing your password will require you to re-authenticate on other sessions. This includes mobiles apps such as the D4H Equipment Management app, the MyD4H app and third parties, as well as other connected services like ConnectRocket.

If you need to (re) create an API key, see How do I obtain an API access key?