Battery empty

Hello all! I left the light on my car and now the battery is empty. It is a Ford Escort Diesel. So then a Mercedes passed by he declared himself willing to help me. We then also wired the cars, but with me nothing moved: no light, no starter, nothing. What could this be? Wrong wired (we were insecure with Mercedes)? Cable broken (we were in the trunk over the winter)? No idea what else it would be like llte (Marder has additionally bitten my cables!??!?!!) Well, yes I am not ADAC member so it would practically not have to call. And since the car is too far away from home a battery charge with charger is unfortunately not in question either. I need it tomorrow at the university. Thanks for help! Mfg Marie