2008 GL450 engine won't start

Somehow my GL450 had lights all on when I returned from the gym a little more than an hour later. When I pushed the start button, it wouldn't start. The message says something like "restraint system not functioning". It sounded like battery wasn't strong enough, so we tried to use a battery booster. All lights were working initially. Later, we tried to use another car to jump start it but got no response.

Initially, the car was responding to my remoting locking and unlocking message. However, after all the jump start tries, it now was totally unreponsive: no lights, not responding to remote key presses. The car now gives out a constant beep, which I can hear slightly even from outside.

I wonder what the problem could be?

Thanks for any help.