If the sleep sensor is showing as offline, there are a few things you can try to get it back online.
First, ensure that the sleep sensor is fully plugged into the wall and that it is properly connected (one end of the USB cable in the wall plug and the other in the sleep belt).
It is also worth making sure that the wall plug is turned on, or the power strip it is plugged into is turned on.
On the sensor itself, a red light will appear once it is switched on properly. If the red light appears, allow 5-10 minutes for it to come back online in the app.