I had a remote die on me once, even though the red LED worked and it had a good battery (tested with measuring blocks in VCDS).
First step would be to change the battery, but apparently you have to have the old one out and new one in very quickly (less than a minute?).
If that doesn't work then try resync the key to your CCM (can be done quickly in VCDS or there's a manual procedure somewhere).
If none of that works then you can pull the remote apart, get the part number of your remote and even replace it with a used one, then resync it
You can still use your key at the minute, but after manually opening the door, put the blade in the ignition to prevent the alarm going off.