Welcome to the community forum elodie32 and so sorry to hear about your problem.
To be honest it’s not looking good, but maybe you could check to see if there’s a Fairphone Angel near you who might be able to help or advise.
It sounds to me as though your SD card may have been formatted as an extension of internal memory, instead of being removable. Are you familiar with the “Android SD card paradigm”?
P.S. - In view of the unstable state of the file manager, it’s possible that in order to get the phone back into a good working state you may have to perform a factory reset. I strongly advise copying any files that you can still access, and backing up or making notes of app configurations, passwords etc.