In some cases, after hours of proper operation, the Thunderboard will stop sending real-time environment sensor updates. You will notice that your Dashboard widgets have values, but they are not changing (e.g. the Microphone volume never changes).
Solution: press the reset button near the center of the Thunderboard. The green LED near the center of the Thunderboard should flash one or two times then go off - indicating a connection.
If this does not resolve the problem, please reference this FAQ: Silicon Labs Thunderboard Sense 2 will not pair