Key fob
 

Key
 

This means that the transponder in the key is not transmitting to the vehicle, setting off the alarm. It shouldn't happen by replacing a battery, but you may need the key reprogrammed. If you have 2 other working keys, you can do this yourself. There is a process that we can give you, but you'll need 2 other working keys. I have seen some fobs that have a chip in them that can fall out when you take the fob apart. Your's probably isn't one of these but possibly? The chip looks like a small capsule and they are just sitting inside the fob on this type. Make sure that one, the battery isn't upside down and check it with a meter to make sure it's got sufficient voltage, and make sure that the contacts are making a good connection between the two halves of the fob. Sometimes all you need to do is bend the tabs very lightly so as not to break them to make good contact between the 2 parts.