Lock / unlock help

Been driving the disco with no probs today but changed battery in one of the key fobs after locking it and gone to go back out and it unlocked ok, was driven and now when you press lock it locks all the doors and immediately unlocks them followed by 2 chirps of the alarm. All doors are closed tight.

Any ideas? Need to get it locked and can only use fob as door key doesn't work.

Sounds like something isn't locking so check each door in turn.

As a temporary measure: tape down the courtesy light switch in the passenger door post, lock the doors from inside, exit through passenger door then lock that with the key.

Why key no work? Need new lock?