Dangerous Things have fantastic aftersales service and a lifetime guarantee on their products.
This is one of ways of contacting them,
or, alternatively on the Dangerous Things Website.
Before that we should help you try and diagnose/remedy the fault before you contact them, or at least get them as much information as possible.
Could you provide a few more details:-
What fault finding have you tried so far?
What have you been using it for?
Access?
Storing NDEF data?
Can you still read it with an app (taginfo etc.)
Is it working intermittently? or not at all?
Has the read range degraded?
How long has it not been working as expected?