Q.lemon tree in pots
I have one lemon tree in a big pot on terrace which gave me 3 lemons but suddenly it bacome totally dry, even no leaves on it right now. What to do? It was 2 year old plant which was flowering wonderfully but suddenly it was all over.

It sounds like the problem is uneven watering. Citrus trees are deceptive in that they will not lose their leaves due to being too dry until after they have been watered. I would make sure that the tree is getting enough water, all the time, especially since it is in a pot and during fruiting. Potted plants require more water, at least twice a day. Even if you water regularly, you may have a dry pocket in the soil that is repelling water. It happens with potting soil if it gets too dry one time, or if you always water the tree from the same spot on the soil. You should try soaking the tree's container in the tub for 1-2 hours, covering as much as possible with water to try to resoak the soil. For additional info, the following articles may help: https://www.gardeningknowhow.com/edible/fruits/lemons/growing-lemon-trees-containers.htm
https://www.gardeningknowhow.com/edible/fruits/citrus/tip-on-water-requirements-for-citrus-trees.htm