My plates got stolen and I didn't know what to do.  Luckily I got my registration renewal, we looked for their number.  They make it very very hard to find however we did.  Mind you we are in Texas so you may need to know your laws, but going in blindly, we contacted them.  They will request for your sticker and plate that remain if you had any plates on your car.  Mine were both stolen and I had a back plate with a different plate on it with ugly old bolts that were not mine.  Tip: learn and check your plates! After calling them I then reported to the police and they will ask name, address, when you last remember seeing your plates.  I didn't notice cause I didn't drive my car often.  I noticed today simply because the front plate was gone and the back looked weird to me. Sooner you catch it the better. The police will not need the plates but it's always better if they can have it and it won't hurt since the County Office may not really need it to issue new plates.  So now what? If you live in an apt let your apt office know what's going on so that they may be aware.  Put your new plates and sticker on the car. It's 6 bucks for you to get new plates and I went a bit further and got new bolts with special tool to take the screw off and on.  Financially it's not really worth it but it may save you the headache of having to deal with all these things.  Oh if you have toll tags you may need to contact them and let them know what happened.  Keep your case number with you just in case. Hope this helps if you get in a bind.

Update: as much as I would have loved to have used these my car did not have big enough holes. I went to pepboys to get some fasteners and they gave me personal one on one help with getting the plates installed.  The manager said however that if your plates are going to be taken it will no matter how much security  you try to put on the plates.  He said it's easy to just snap the plates off with a big pull and they wouldn't care about the damage that happened to your car.  So just know that no matter how safe you want to be it could happen.