First think I would try is get some graphite lock lubrication and apply it to the lock mechanism, make sure it come move. If that doesn't help then it may be the inside door handle locking rod has come detached from the locking mechanism. The only way to know for sure is to remove the door panel and look inside the door. There is a locking rod coming from the outside door handle, the inside door handle, and one from the inside lock button as well as the door lock actuator.
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