Why can't I see the blue/red light anymore when I press the button?

Most likely your battery has gone flat

To replace the battery, please follow the steps below:

First you will need to purchase a replacement battery - CR2032 is the battery type required

Remove the wireless sensor cover gently, using a flathead screwdriver

Remove the dead battery

Place the new battery correctly into its housing

Be careful not to bend the positive and negative contacts

Put the sensor cover back on the sensor

Now when you press the button you should see the blue/red light appear