Character falling through terrain

I know there are a lot of similar questions but I read dozens of them and I'm still stuck. Be patient: I'm a newbie and English is not my native language. So I was modifying the terrain today, and my character suddenly began falling through the terrain when I hit play. It already has a capsule collider, which is not trigger, and a rigidbody, which is not kinematic. The terrain has a terrain collider. I don't know what to do, any help would be great!