Colliders have stopped working?

I’m using the standard assets 3D character and he keeps falling through the floor. Both objects have colliders, and no matter which object I use he falls through it. I tried attaching a rigidbody to a table and it fell through too. I have no idea how to fix this problem.

The only thing I think it might be is that I installed the free 3D game kit, but even after I deleted it this is still happening. Help?
6105342--664476--upload_2020-7-18_20-26-55.png

Does either object have a Rigidbody? What layers are the objects on? Is either Collider set as a Trigger collider?