You have locked yourself out, here’s what to do:

Approach the caretaker

If the caretaker is still in office, or if you can wait for him to get to the office (e.g. you stay at a friends place), you can approach the caretaker.
The caretaker has a key that works for every apartment, and he can let you back in.

Call a key service

If you need to get back into your apartment as soon as possible, or if you don’t have a place to stay, you can call the key service.
There are a few key services in Frankfurt (Oder), of which one is:
Sicherheitstechnik Menge:
+49 335 50 46 54
After 6 PM: +49 335 40 15 27 44
But I never recommend to call the key service, as its very expensive (especially during late evening or weekends); So this should only be the last resort.

Do it yourself or ask for help in the group

If you have some time and don’t want to loose all your money, you can try to open the door yourself or ask for someone in the group.
There are tutorials on the internet on how to do that.

I have lost my keys completely

If you have lost your keys completely, its gonna get expensive.
In that case you have to reach out to the caretaker and he will get you some new keys.
You will have to pay for the new keys.

Always make sure to take care of your keys and never forget them.
One advise that I can give is to lock the door from the inside, this way, its impossible to forget the keys.