Character falling backwards!

Hi, kinda new to unity and have made a terrain, 1st person character and tree. character = rigidbody +box collider, tree = box collider. When I walk into the tree the character falls onto its back and cant get up.
Really need some help and would aprisiate if you could answer why and have a solution.

(sorry for the bad spelling)

Select your character then in the inspector under rigidbody expand constraints, check freeze rotation x and z. This will stop character from falling.