3d Car model continuously falls down, when I apply rigid body component.

Hi all,
I'm in a trouble and trying since last 30 hours, I have done different experiences and applied many concepts which I learnt through google. But I'm unable to resolve my problem..
The problem is
When I add rigidbody component of any 3d model, like suppose 3d car model, when the game starts, the model falls down continuously. The model is place above the road (y > 0) and road (terrain) is placed at y = 0. Please help me out, what is the problem and which thing I'm missing.. thanks in advance

 

Make sure your road and car has colliders and they are not triggers.

Also check your layers and check if they have intercollision enabled via Project Settings -> Physics -> Collision matrix.

 

Thanks dear, problem is solved
Also please if possible, guide me regarding following error
Non-convex MeshCollider with non-kinematic Rigidbody is no longer supported in Unity 5.

 

Enable "Convex" on the Mesh Collider.
Also, I suggest using primitive colliders. E.g. boxing it out because it yields better performance.