I am having severe left heel of my foot after rest? Why ? What is the treatment to be given?

If you do a lot of walking on hard surfaces, or are a runner this is not an uncommon condition and a general term is Plantar fasciitis. Most people don't give this condition enough time to go away but if you keep off it for a couple of months, it will heal itself. If you want a better diagnosis see a podiatrist who will probably recommend a heel pad for your left shoe.