At least with an N9 there's a trick that sometimes works, not sure with 770 though. If the battery is totally flat and wont have juice to boot up to charging state, and also not possible to use the flasher charging on it, just plug it on and off the charger many times and it just might get trickle-charged in the process far enough to be able to boot to charging state. You need to do it many times, tens, maybe hundreds even. Plug charger in, when it flicks the lights and dies, remove charger and plug in again... and so on. Eventually it might start charging.