Do you mean that you can't even start the car, or that you can't open the doors remotely?
If it's the former, you'll need to go back to your dealer and have your keys reprogrammed.
If it's the latter, you should replace the batteries in your key fobs and then try following the procedure outlined on this link:
http://www.mazdaworld.net/forums/index.php?showtopic=1140
This should let you reassociate your fobs with your car's keyless entry system.
Good luck!
Steve, July 2007