renualt megane 1.6 my key does not turn off the imobilizer

how old is the car? what type of key?

l presume the sensor for the immobiliser, is located above the car is rear view mirror?
You would likely have been placing the key against the sensor in an attempt to deactivate the immobiliser, before it packed up altogether.
Take out the single screw & carefully open the key fob. Replace the 2 batteries (should be type CR2016), WITHOUT touching the new ones with ur bare fingers (this shorts the batteries out) & reassemble. If this fails to make a difference, open up the key again & check the soldered ends where the small black sensor is joined to the circuit board. These joints r prone to becoming detached, requiring re soldering.
Good luck!
I