Audi tt 2002. Keys wont work remotely or turn ignition over

Hi - I recently had my car recovered to my house. Gearbox problems. It's now been sat in front of my house for about 3 weeks. I've turned it over a couple of times. However,  today the remote keys wont work to centrally unlock, and the ignition wont start. Nothing at all. No interior lights come on either.. could this be a battery issue? I have tried to re-sync the keys. Be great to get some help, as I was just about to get the gear box sorted. I think I heard the car alarm go off this morning - but can't be sure it was mine - so the other thing I'm wondering is if the immobiliser has kicked in. Is there a way for me to assess and work out what's happened? Thanks in advance - at my wits end! James