I've had this same issue (well, key stuck in the lock while unlocked) which occurred after my other half used her key to unlock the door from the outside while my key was in the lock on the inside.
I did the same as Neil / @spiralarms and took out the barrel - using the two keys to gently twiddle around and get the alignment back in order.
Always worth trying to fix these things yourself; would have felt silly calling out a locksmith for such a simple task
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