If it's totally flat then you'll know it since your key remote won't work and you won't be able to open the trunk from the trunk handle switch. Your only entry into the car is from the driver's side door using the actual key. So go ahead and lock it, as long as you have access to the driver side lock you can get into the car.

Connect it up to a charger and see what happens after 24 hours. Or else get a jump start and go for an hour drive to charge it up. Then when you park it, connect it to the charger to top up the battery charge level.

BTW, is this the 2003 X5 and is it on it's original battery? If so 7 years is pretty darn good.