Most likely your battery has gone flat
To replace the battery, please follow the steps below:
1️⃣ First you will need to purchase a replacement battery - CR2032 is the battery type required
2️⃣ Remove the wireless sensor cover gently, using a flathead screwdriver
3️⃣ Remove the dead battery
4️⃣ Place the new battery correctly into its housing
Be careful not to bend the positive and negative contacts
6️⃣ Put the sensor cover back on the sensor
7️⃣ Now when you press the button you should see the blue/red light appear