What would happen to the enzymes activity if denaturation occured

The enzyme activity would decrease and even completely stoped.

When enzymes are denatured, this are usually due to the extreme pH environment or high temperature than their original optimum.
When denaturation occurs, the active site would change shape. This will lead to that the substrates can no longer bind into them therefore the enzymes can no longer work.
Usually when the active site starts to change shape, the rate would be slower, but when it fully denatured, the activity is zero.
Therefore if this happens, the enzyme activity would decrease and to the place where it stoped working.