Why won't my car doors lock?

Got a Nissan Micra. I just noticed this morning that the doors were unlocked :-S. I tried to lock them but they kept on unlocking themselves. Tried to lock by key and by the door lock but it still unlocks itself. What's happened? I've only had my car washed recently..could that have messed it up :-S
Update: yeh fixed now..thank god lol. Slammed it shut, seems to have done the trick. Don't think I did it hard enough the 1st time. Thanks all!
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