The shape of an enzyme is important because enzymes are shape specific. Each enzyme can only react with one kind of substrate, and the enzyme's shape determines which substrate that is. They fit together like a "lock and key".
If an enzyme is denatured (or its shape is altered from its active form), then it will not be able to bind to its substrate and the substrate's activation energy will not be lowered, usually resulting in the cessation of whatever reaction the substrate is undergoing.