Last time my car batt flat also can on radio, alarm working, light still can turn on but cant start. The battery already cant handle the starting load. After change then ok already.
To be sure batt dead/weak, just measure with multimeter. 12.7V is fully charged, 11.7V is dead . You also can do load test when batt is full ~12.7v, measure the voltage drop when starting the car. It shouldn't plunge below 9.5v

My car blm2008, full at ~12.7v, 2days no drive it already dropped to 12.4V, about 30% drain. Doing a load test it only plunge to 10.xxV. 2years old maintenance free batt. Just did own periodic check since maintenance batt cant last >2years.

How about other electronics? working? e.g radio/light?
Check batt connection at the engine bay. Very common for proton the battery connector not tighten properly.