The battery is not properly fitted in the battery compartment or is depleted.
Check whether the battery is correctly inserted in the battery compartment.
If necessary, insert the battery correctly.
If the battery is empty, replace the battery as follows:
One 3 V battery (CR 2032) is required.
Depending on the model, open the cover to the battery compartment:
Undo the fixing screw on the battery compartment cover using a cross-tip screwdriver and then remove the cover (top figure).
Unlock the cover, e.g. using a flat-head screwdriver (bottom figure).
Replace the battery with a new one. Make sure the polarity is correct.
Refit the cover.
Depending on the model, tighten the fixing screw.
Dispose of the old battery in a designated recycling bin for batteries.