Yes your situation can be upsetting and I’ve seen others have this issue.
How much data will you loose by resetting the phone and reformatting the SD card as external?
When you say “most part of may data was stored on the card” do you mean previous to insertion or data moved from the phone to the card?
It appears however this is not a Fairphone issue but more a lack of clarity of Fairphone and lack of knowledge on the user about the benefits and drawbacks.
I didn’t even consider formatting the SD card as internal as the one I was using was from another phone and contained data I wanted to keep.
The issue for those that choose to use the SD card as internal storage and then have problems is an Android issue which you see from the following link. The method suggested for you may also seem daunting and may not work but may be worth reading.
What would happen if I remove a formatted SD card?
If you format an SD card as internal storage then remove it, applications will lose access to data which leads to considerable disruptions in operations. Fortunately, devices store information regarding the formatting of SD cards on internal storage. Therefore, as soon as you reinsert the formatted SD card, your phone should go back to normal.
How may I recover data from a formatted SD card that becomes unreadable?
When formatted SD cards refuse to be read, the best options for owners of devices that run on Android would be purpose-design repair applications. Such applications would allow the recovery of important data inside corrupted cards and they could restore the card themselves too.