Why does denaturing by temperature or pH make an enzyme non-functional?

Active site of an enzyme is fixed for particular substrate, or you can understand it better via lock-key theory, in which substrate acts as a lock and active side of the enzyme as a key, and you know well that each lock has a specific key to make it unlock. So if active site of an enzyme is altered due to any reason, it can not be fixed into the its particular substrate, and functioning of an enzyme obliviously requires a substrate.