The two main causes of an inactive water valve are either a low battery or a Wi-Fi connectivity issue.
If your battery indication is low, then please move the valve to a more sunny location. The valve should reconnect automatically once the battery is sufficiently charged. If the battery was critically low then the device may have turned itself off and will need to be manually turned on after a period of charging.
If there was a temporary Wi-Fi outage, then the device will automatically reconnect in approximately one hour. A constantly absent Wi-Fi signal will prevent the valve from reconnecting. In such a case we recommend either getting a Wi-Fi signal amplifier or moving the valve closer to the router.