What happens if you don't pay your credit card debt in Germany?

What will happen?

The bank can sue you and get a court order to seize. And AFAIK, they are allowed to count any incoming money to that account towards payment of your debt as long as the account is not under special seizure protection (see below).

I agree with @Aganju that the Schufa entry will be a problem - though IMHO there are worse things than someone who "spent my money right and left very stupidly" not being able to get further into debt via furniture or car loans. I don't see mortgage on buying a house as relevant for the 3 years after you've finished paying back your debt (when that entry can be deleted).

The big difficulty will be that the bad Schufa entry will hamper your ability to rent a room or flat. Part of the mechanism for bad Schufa entries is that you did not reply to the bank contacting you about your debt, which is read as unwillingness to work at a solution.

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What to do?

• Have you been to Schuldnerberatung (debt counseling)? https://www.meine-schulden.de/ may be a starting point to find a trustworthy non-profit offering debt counselling nearby.

  Before contacting the bank, I'd go there and listen what they say. They can tell you what you can usually negotiate with your bank (e.g. a payment plan that avoids the Schufa entry). Since that is urgent now after already 3 months, you should get an appointment with the Schuldnerberatung immediately.

• You'll need a so-called P-Konto (Pfändungsschutzkonto) which has a monthly limit that cannot be seized (currently 1180 €), so you'll be able to pay rent etc. from social insurance money.
  I cannot recommend whether you better ask your current bank to convert your account or get one somewhere else. The Schuldnerberatung will have recommendations for that.

Normally, the bank has your employer data (they see the incoming salaries).

They will try to work with you on a plan how to get out of the hole, but as a last resort, they lock your account until the salaries / unemployment pay have paid the open bill, or - if that won't work, or you move your income to another bank - they contact your employer / the government to garnish your salary / unemployment income.

Long-term, the Schufa entry is the bigger issue, as you will be unable to get any loans - for furniture, car, a house, or whatever; nobody wants to lend money to someone who is marked as 'doesn't pay back'.

Contact then proactively, explain the situation, and ask for help to get it back to normal. You are not the only one in trouble, and they will be willing to help you.