There's no need to panic; the data is not lost just because you forgot to activate the sensor. The sensor continuously sends data, but it's not displayed in the system until it's activated. 

You can activate the sensor in hindsight by using the "Sensors" tab and entering the respective sensor ID. During the activation you can set the activation date to when you placed the sensor in the ground. 

Please remember that you also need to deactivate the sensor. The deactivation date will be the exact day and time you perform the deactivation on the Valenco platform. You can change both the activation and deactivation date by following the article "Can I change the activation date of a sensor".