What are the potential problems?
The battery was probably drained. This can happen for several reasons, one of which is the alternator which recharges the battery has failed. In that case the battery will not get charged and will slowly drain again. You can check this by measuring the voltage when the engine is running. If the battery is getting charged the it should read just over 14 volt. (That is assuming you have a meter for that on the dashboard. For some reason manufacturers tend to skip on this very useful device in several models).
Alternatively your battery might get charged just fine when the engine is running. But if you leave on something which uses electricity when the car is not running then it will drain overnight. Classic example for this are the headlights, but given enough time lesser users such as the light in the trunk will also do the job.
The easiest way to check for these is to put an amperage meter in between your car battery and the rest of the car. (Caution: Do NOT attemp to start the car or use massive amounts of electrical power while such a device is in place. It will likely overwhelm it and you will see the magic smoke escaping.)
Alternatively you might be able to do the same with the fuse box, by checking the power drain across each fuse individually.
Same warning: Do not try this for massive power users. E.g. do not try to start the car while measuring the fuse to the starter engine.
what should I do to prevent this from happening again?
Finding out what caused it in the first place is the prpoer solution (and ofc. fixing that). Alternatively you can disconnect the battery each time after you have parked. I've done this in the past. It is a PITA but it works and it can be a big help if you need to bridge a few days until you have time to properly fix it.
Lastly, not asked, but:
If it happens again you can jump start the car from another battery. No need to call ADAC for this as long as you have a second car (e.g. from a helpful neighbour). As long as you have jumpstart cables this is trivial to do.