In that case, you need to report the plate as "stolen, lost or destroyed" and immediately surrender the registration. DMV will issue you a new plate and registration.
What if I'm in an another state? Would it be a problem? Or can I do the whole process by mail as if I was here?
You have to go to that state's DMV agency and surrender them there, but you can still surrender them by mail. Just make sure, you are in that specific state when you are in the DMV website.