like #rutter said, have you check if there is a rigidbody attached to your game object and if there is one is it the 2d rigidbody. if you dont have any rigidbody2D attach to your game object you can add one by going to Component → Physics2D → Rigidbody2D, if what you need really is a rigidbody then go to Component → Physics → Rigidbody. ohh and make sure you have the gameobject in question selected while doing so otherwise you wont be adding it to the correct object.