Upgrade to Buster failed, no network, NC configuration seems to be lost, how to rescue data

to be honest, I am not that Linux Guru, that is able to solve every problem. The oposit is the case, to get NextCloudPi up and running for my family to have our contacts and appointments in our responsibility was the biggest things i achieved in IT. But now my cloud crashed after a failed upgrade to Buster and i was to lazy to set up backup functionality. My steps to get NC running again by following this instruction made the things even worse (if i would have followed this instruction at the beginning, the upgrade would have been surely successfull). Now I have no network interface recognized on the pi, the NC configuration seems to be like after a fresh installation, no ssh. So I have to face a new installation of NextCloudPi. But what about my date? There were only a hand full of documents, no pictures, only contacts and calendar entries. Now my questions:

  1. By setting up a new NCpi, having my contacts and calendar entries in our smartphones, by an initial syncronization with the NC on the first start, will we have out contacts and calendar entries again in the clout from our smartphones or will the data be erased on the smartphones because the nc is empty?
  2. how can I check if the contacts and calendar entries are still in the database manually?
    2a) If the data is in the database, how can I backup this database manually and use it again after setting up a new NCpi?
  3. Where can I find the documents I saved in the nc?
  4. Is my way to try to resque my data, set up a new NCpi and migrate the data back into the new NCpi a good strategy or is there a better way to get back to a running NC with my data?
    Thank you for your answers in advance

This “pi” thing is a toy-grade SBC with primary storage located on an SDCARD. For the love of everything that makes any sense in this world, why wouldn’t you back it up before mucking with things you don’t understand?

As for retrieving the data, pull the sdcard out, plug it into your laptop, mount it, and search for the files you need.

Your answer does not help me much. If I do not know, where to search on the SDCARD (which path) I will hardly find them. How are my calendar entries and my contact organized? Are they in the mairaDB? How to enter the db? How to check if the data is still there? How to backup the db manually to make sure the migration to a new ncp instance will be successful?

The default path to data is:

Default location of database is;

Thank you very much for the paths. Is there a default technical user for the database and a default password?

No default, the password is generated during first boot and stored in NC’s config