The Body control unit (BSI) on Citroens (and Peugeots) sometimes goes batshit-insane and switches things off for no other apparent reason than it felt like it.
A reset is usually enough to resolve, but the steps need to be followed almost exactly, and the car should be thoroughly checked afterwards to make sure everything is working.
This documentation details how to perform the reset
To perform the reset;
- Put the drivers window down and lift the bonnet (leave the key in the ignition)
- Ensure all equipment (stereo etc) is switched off
- Ensure all doors are closed and reach through the window to remove the key from the ignition
- Wait 3 minutes
- Disconnect the battery and then wait 15 seconds
- Reconnect the battery and wait 10 seconds (Do not open any doors)
- Switch on the ignition (but don't start the engine) and check that all tests pass
- Hold the lock button on the key down for 10 seconds
- Remove the key, open the drivers door, close it and check the central locking works (I had to hold the lock button down for another 10 seconds at this point)
- Start the engine and complete the systems check
Note: When you put the drivers window up, it won't automatically go all the way up. Make sure you close it all the way before putting it down, otherwise the automatic close won't close it all the way until you next turn the ignition on and off