My sensor will not turn on

Read the light first

Press the button once and watch closely. Three fading red pulses followed by darkness means the battery is too low to start, which is expected behaviour rather than a fault. No light at all usually means it is completely flat.

Charge it properly

Put it on the dock for a full hour before concluding anything. A deeply discharged battery can take several minutes on the charger before it even shows the charging pulse.

While you wait, rule out the dock. Try a different USB cable and a different power source. A dock plugged into a laptop port that has gone to sleep will not charge anything.

Clean the contact area

Rinse the sensor and the dock recess with fresh water and dry both. Dried salt between the two is a genuine cause of failed charging.