From your picture it seems you are missing the cover of the key cylinder. But that doesn't look like somebody would have tried to break into your car. That would look worse then.
Not sure if you can replace that cover only - I am more under the impression that you might need a new lock cylinder - and that is something which can only be fixed by your dealer. Not that replacing the cylinder would be a big deal - but buying one that fits to your key......

Guys: please correct me if I am wrong.

Good luck,

Helmuth

EDIT: as Qsilver7 already wrote ...........