The sensors are technically always "Active" (unless the battery is removed) but are shown as "Inactive" on the web site if they are out of wireless range for more than 24 hours. To re-activate, move the sensor and gateway closer to each other or position an additional gateway in the center of the "dead spot" to get more coverage. This is similar to a cellular phone network in that if you need more coverage you can add additional gateways. The rule of thumb is about 200 feet of range in a building environment. For maximum wireless range make sure the sensor antenna and the gateway antenna are both pointed at the sky and that the gateway is not next to any metal obstructions such as filing cabinets etc. Sometimes you will see sensors go inactive because the gateway they are talking to has lost its network connection in which case you should refer to the troubleshooting guide for network connectivity issues.