Suddenly the power steering doesn't seem to be working. What could cause this?

Rain making it better sounds like a lubrication issue. Check the lower ball joint boots. Many times they will tear allowing the joint to rust. This can make steering effort higher. Rain might temporarily lubricate the joint and ease the effort. It should be easy to check, Remove the outer tie rods from the knuckles and turn the wheels to check for excess binding. Good Luck