If nothing else works, there is always a chance that the receiver inside your car is broken or disconnected. In that case, you’ll probably have to take your vehicle to a professional.
The other option is to buy a replacement remote, which you can obtain either new from your local dealer or used. If you get a used one, you will have to reprogram your vehicle to recognize it before it will lock and unlock your doors. So if you discovered in an earlier step that your car uses a remote that one can’t easily reprogram at home, keep that in mind.
Used car key remotes are typically cheaper than new ones, but costs associated with programming may outweigh the savings.