Check the battery in the remote first. Just pull it out and use an (assume 3V) cell to test it with, even if you have to use pokey wires to make contact. 2x AA batteries will do the job if you need, or dig into the kitchen scales, bike computer, TV remote, etc, to find a CR2032 or whatever matches the remote's battery.
It's quite possible that the thing has done a panic and locked the immobiliser. You would be advised to work out what model the alarm is and google the instructions to find out what sort of rain dance you're supposed to do to fix it.