If your Fitbit has stopped functioning correctly, please get in touch with Fitbit's international service team. They'll be able to assist with any troubleshooting.
If your Fitbit has broken and is within the 12-month warranty period, the best thing to do is to take it back to the retailer you purchased it from. They'll be able to resolve your problem and get you back up and running. If you've purchased your Fitbit from Fitbit's online store, you'll need to make contact directly with Fitbit as they are your retailer.
If your Fitbit is no longer within the 12-month warranty, please contact Fitbit's international service team.