Programming

is it possible to extract a c program from its exe.file (executable file)??
if yes how?
if no why?

Not exactly. As it was already mentioned when you compile the program it is "converted" to machine instructions. From this point you can' restore the original source code - it's just not possible.
However you can see the machine code and realize what the program does at low level. If you are skilled enough this can help you to figure out how something works. This is called reverse engineering. I'm not going into details, because I'm not an expert in this area.
The whole process can be compared to a cake baking. You have the products, you mix them, bake them and finally you have a cake. You can't restore the products and determine what they were. However if you are skilled enough you can guess them from the taste