Button hooks can be mde from coat hangers, but they won't keep ther shape. Used for opening warded locks and some 2 or3 lever mortice locks.

Lubricate the lock, put in hook in keyhole like an upside down key, move it up and down until you feel the levers moving do this a few times then when you are on the up stroke turn quite firmly, try the door this often this works without you realising.