Unfortunately when this happens it normally means that the remote has failed. These remotes can normally go several months without a battery without losing their memory.
Never the less, it is possible that it did just lose it's memory while the battery was dead, so before investing in a new remote, you should try reprogramming the remote first. Dealerships can do this very quickly with a special tool for about $45, however if you are handy around a car it is something you can do yourself as well if you are careful.
To program the remote yourself, you will need a piece of jumper wire or a paperclip. This will be used to short pins 1
of the OBD connector under the driver side of the dash. Pins 1 and 4 are illustrated here:
It is absolutely paramount that you get these pins correct! If you short the wrong pins together, you can do serious damage to the vehicle.
1) Short pins 1 and 4
2) Close all doors
3) Turn ignition switch to ACC then back to OFF
---The door locks will cycle once indicating you are in program mode. If this does NOT happen, there is another problem with your wireless system---
4) Press the Lock button on the remote once, then within 10 seconds press it two more times.
---The door locks will cycle again indicating the remote was added to the system. If this does NOT happen, there is a problem with your remote---
5) If you have a second remote, repeat step 4 with it... each time you go to program mode all remotes are deleted so you need to do them both.
6) Remove the ignition key, pull the jumper wire, then reinsert the ignition key and cycle it once to end programming mode.
If all that happened was the memory was lost, that will get you back up and running. If you can not get into programming mode, the vehicle will need to be checked out to determine the problem with the keyless system. If you can get into programming mode but you can not add the remote, the remote is faulty and will need to be replaced,