This is interesting because the same thing has just happened to me, car was fully charged during Friday night, unplugged and left sitting on the drive all day Saturday, then at the most inappropriate time of 2am Sunday morning the alarm siren was going off, i had to go outside to try and stop the noise, remote barely operated the doors but i eventually got them open but the car was completely dead nothing on the screens etc. locked it back up and went to bed, luckily no more noise from the alarm. (Apparently the siren was the power sounder, designed to operate if the battery is disconnected while the car is not switched on or not unlocked, i.e. someone trying to steal it) and in my case the car thought the battery had been disconnected because it had gone down below a certain level of charge.
Sunday day time went out to the car neither remote would open the doors so i concluded that the car was not interacting with the remotes, put the key in the drivers door and opened it with ease, opened the bonnet and connected a battery charger to the terminals in the engine bay as described in the manual, once that had received a bit of power i could open the boot to get to the battery, connected the charger directly to the battery and have left it charging over night. My intensions are to periodically check the voltage of the battery to make sure its holding charge otherwise i'll be buying a new 12v battery.
All this proves is that your 12v battery needs to be in tip top condition for the car to operate correctly. And at this time of year it's hard on any car battery, so maybe a periodic trickle charge would ward off electrical problems with your Ampera?