Collisions with terrain not working.

Okay, So I have a rigidbody that consists of many shapes, and I have a trigger where it collides with a cube, which works fine. When I try to get it to collide with the terrain nothing happens - the Rigidbody collides and is stopped by it, but in my script, there is no detection of that.

void OnCollisionEnter() {
if (rb.CompareTag ("env")) {
spd = 0;
thud.Play ();
hitter = true;
}
}

None of this code executes when I collide with the terrain. The player is a rigidbody, it moves with force and floats above the ground. Nothing I have found on the forums has helped, so if you have any ideas please let me know ASAP.