My Fitbit has stopped working- no lights nothing can it be fixed?

plenty of issues could be going bad on the device.

-Battery,

-Motherboard,

-Led Panel,

-Charge port.

Assuming you have tried a new charger in the device, i would take it to an electronics repair shop for a proper diagnostic as to which one it is, as it is impossible to tell without opening the device and testing it physically.

Costs of repair will depend on many factors

-What is failing

-quality of parts used

-quality of technician doing the repair

-how much the technician feels the labor should cost.