If the latch is locked, the button moves effortlessly when pushed. You will feel pressure and it will definitely be obvious when it releases with a low click. Likely one of the linkages (and there's a BUNCH of them) in the latch mechanism has come loose so either it isn't unlocking or the button no longer activactes the lock. Either will produce the same result. Make sure the inside lock plunger is moving coreectly and you can hear the lock click when it is pulled up to unlock or pushed down to lock. If that is working, it has to be the button linkage.