If your ADT doorbell is not working, the first thing you should check is the battery. To do this, open up the doorbell and remove the battery. Inspect the battery for any corrosion or damage. If the battery looks fine, try charging it for a few hours before reinserting it into the doorbell.
If your ADT doorbell still isn't working after checking and charging the battery, then there could be a problem with the doorbell itself. In this case, you'll need to contact ADT customer service for further assistance.