Don't worry,  as you may already know: "normally" a trailer does not need liability insurance, so DMV will not suspend your driver license or registration because your plate got stolen, and therefore you will fail to surrender it.
So, just to make sure: do I need yo surrender trailer plates?
You must surrender your plate if your trailer doesn't have a valid registration, it's ok if you "lost" it as long as your registration is valid.
Ok, thanks for that! Btw, my girlfriend wants to surrender her plate, but she's very new with this stuff.. Could you send me some information on how to surrender plates by mail? So that I can send it to her, please,