Suspect it's just stopped working like broke. The illuminated red light on the fod means battery OK. When they go flat that light never comes on. Observed that one. You may or may not get the dash low battery warning but the fod won't light when you press a button.
The one where people don't take off the protective film / paper changing the battery you wouldn't get the illuminated light trying it out before you go to the car. You can give the battery a rub with a micro fibre cloth if it one that's got bitter bite coating as some are said to have which need a wipe.
Could be a duff battery why it's not lighting up now but given it was lighting up before but not opening the car, the first instance feels like a failed remote (try the other). If the second one doesn't work then it's going to be a failure in the car of Kessy, central locking etc. A dealer thing.