The temperature gauge inside my car is reading -40.

Is this a simple fix. Such as unplugging the battery. Or something a little more serious.

Thanks.

Check the outside air temp sensor clipped behind the front bumper lower grille, sometimes they get damaged or become faulty giving false/default reading.

Note: Description is from USA Mini so passenger side in UK!
http://i555.photobucket.com/albums/jj470/mab01uk/temp-sensor.jpg