Hello Vasu ,

They are two different questions " why our earth is not plane " and "E=Mc^2"

Because E=Mc^2 is theory of relativity.

The E stands for energy , the m for mass, and the c for the speed of light (approximately 300,000 kilometers per second).

It means  that there is a sense in which mass and energy are the same thing. There are ways to convert from one to the other, and vice-versa. It also means that the amount of energy in even a very small amount of mass is enormous.

And for the question why our earth is not plane

If you want a theoretical answer then there are various answers like :

1. By the Stars
2. Shadows of Sticks
3. Watch an eclipse

and many more.

If you want any further elaboration of the answers please mention in comment section.

Hope it helps!