Mini Cooper S 2014 won't start and makes one single clicking noise everytime I push start button.

if car been sat for 4 months and battery went flat chances are its battery,, or might find the programming has lost its self for the battery ie battery needs code doing with diagnostic's tool ,, this will ensure the smart charge system is working correctly to that battery would be my first approach, then would check voltage when car running should see 14v to 14.7v fully charge the battery leave standing for a hour then test the volatge new battery will have 12.8v used but ok battery 12.6v 12,2v and under is a failed battery,, also can read voltage when try to start car it should stay around 12v to 12.8v , if the ecu and bcm sees under 11,9v volts it will not power up correctly if get under 11v voilts will throw all kinds of fault codes that are just due to lack of power to run a system but just enough power to store a fault code eml light