Hello @Tracey_Campbell
Welcome to the PurpleAir community forum.
There is a tap feature of the Zen that can be accidentally turned off. It is possible that something may have caused it to turn off. Double tap the sensor and it should come back on. Doing a successive tap can alter the brightness as well. If this does not work, let me know and we can see where to go from there.
For more information, check out our article on the double tap feature found here: Double-Tap to Change LED Brightness