Seized & Jammed Lock Opening: getting ready

The lock works, but only if you lean on the door with your hip while turning the key. After a week of humid weather the bolt is jammed against the strike plate, and without that push the key stops halfway.

Worth answering before the call

Is anyone inside, and can the door be opened from that side? Has anyone sprayed oil or another product into the keyway? Do you have a spare or original key you could try?

My key is worn. Would a new copy have prevented this?

Possibly, if the key was part of the cause. A worn or bent key lifts the pins short of the shear line, so the plug binds even in a healthy cylinder, and a copy made from that key repeats the wear. A key cut from the code or from a crisp original is the fix. If the lock was also gritty or the door had moved, the new key would have eased things without settling them, which is why the locksmith checks all three.

The thumb-turn is stuck from inside and we need that door to get out. What should we do?

Use another exit for now and call. From the inside the fault is usually a bolt pinned by the frame or a bent spindle, both of which the locksmith frees without damage. A main exit door should open from inside without a key or special knowledge, so the repair leaves the door doing exactly that, and if the hardware on it was the wrong type for an exit, a thumb-turn deadbolt is suggested.