So I have a 2002 A8 4.2 Quattro, battery was replaced around a year ago but has needed a jump start a couple times recently. Car isn’t used much. 
Went to use it the other day and can’t get it to unlock. The buttons on the key don’t work and putting the key in the door doesn’t work. Tried two keys and replaced batteries in one of them. Boot doesn’t unlock either. No flashing red light in the car suggests car battery is dead but I thought the door should still unlock with the key. 
 

Any suggestions besides smashing a window (as a garage said to do). 

Hello Nicholas, 

Thanks for being in touch, and sorry to hear of your issue.

It does indeed sound as if your year old battery is defunct, and/or the car has developed a parasitic drain. 
The car should unlock with the key, and since it doesn’t, I would be tempted to give it a generous spay of WD40, leave it for 10 mins and try again following another squirt. 
Be careful if the lock has a sliding shutter across it, since these have a habit of trapping the small plastic tubes attached to the WD40, sometimes resulting in a half day out with a locksmith! Carefully slide and hold the shutter open with a small screwdriver or similar - with respect for slipping and scratching paintwork! 
Perhaps you could let us know how you get on.

Kind regards,

Gareth.