You can manually bring the exterior hand back to the resting position by prying down on it gently with a screwdriver, plastic trim tool, etc. I had this issue back when I lived in a place that got rain and snow. The hand hinge gets rusty and seizes.
If the handle is held in the "open" position, the door won't unlock. Once you get it back to the resting position, you should be able to unlock and open the door. Then take the exterior handle apart, clean it up, and lube it. If I recall correcly, I've had to replace the pivot pins on at least one of these to get it working smoothly again.