2 Pin Reed Switch Normally Closed Effect using BC547
seeing as you want to run this from a coin cell, using electronics to invert the operation of the switch is probably not going to work well.

That leaves you with two options either use a reed switch that turns on in the absense of magnetic fields - a normally closed reed switch, or a dualr throw reed switch;
or use a second magnet to convert your read switch to normally closed and then use a magnet on the door to cancel the second magnets effect (allowing the switch to turn off)