2004 Citroen Berlingo key won't go into lock

Steering lock , door lock , boot lock ?

Oops sorry, driver’s door lock.

It's usually caused by dirt in tumblers of the lock barrel . If you get WD 40 or similar with the tube in the lock and squirt to wash out whilst putting the key in / out quickly , it often clears , But you may have you do it a good few times and have a bit of patience and use plenty of WD40 and may see the dirt and grime getting washed out

I am a great fan of WD-40 but unfortunately it didn’t make a difference.
I wonder if it’s something to do with the central locking. Drivers door - key will go in 20 mm (not 5 mm as mentioned above), no further and won’t turn..
Passenger door - the lock works. Boot - the key goes in but won’t turn.
I was away recently and didn’t use the car for two and a half weeks. Much to my surprise the battery was completely flat, but fine again after being charged up.
I wonder if it’s something to do with that. The power steering hasn’t worked since either, and I’m just about to post a problem. Thank you for your help.

You could check the relative fuses or another key

https://fuseandrelay.com/citroen/berlingo.html