I stupidly, but in error left my lemon tree outside over night when it was really cold and now all the leaves have died and are starting to drop off. If I scrape the bark on the trunk it is still green. Is there any hope?
One night of cold exposure won't have killed the tree, but it will cause death of any leaves present. Now you just need to wait, keep it in appropriate temperatures overnight and it should put out new leaves, eventually.