Hi,
My phone is stuck
with that message :
can’t load Android system. Your data may be corrupt. If you continue to get this message, you may need to perform a factory data reset and erase all user data stored on this device
Can I recover without losing my datas ?
Hi and welcome to the forum.
Could you indicate which operating system is installed on your phone (stock FPOS Android, /e/, Lineage, …) and if possible which version, or the approximate date of your last update?
Please also confirm that you hadn’t previously unlocked the bootloader (unlikely but there’s always a chance).
Unfortunately it’s fairly unlikely that your data could be saved, that’s why it’s so important to have backups, at least of files and other data, if not settings.
it’s /e/
I had messages about lack of storage space, dunno if it’s the cause
Since it’s posted over there, too, we can assume /e/OS.
It is likely the issue would be storage space is full, which Android notoriously lets happen and then demands a factory reset.
A topic about using ADB with the phone in recovery mode to free up space was hinted at already … I didn’t quickly find a ready-made tutorial, though …
The idea is to mount the phone’s filesystem in recovery mode even if user data is encrypted. Since file encryption is used for a while now in Android, the filenames and the file contents are scrambled, but the file sizes remain visible, so large files can be identified for deletion to free up enough space to let the OS boot again. Furthermore, ownership properties may stay visible, too, more easily revealing large media files.
what is bootloader please ?
You can look that up 
Moving forward, AnotherElk’s advice is in my opinion the best approach. You may need a little practical help, though.
It’s possible there may be a fairphoneangel near you who could offer guidance, check out the map.
This would be great. In case Fairphone Angels can’t help, however …
… I guess user tcecyk in the topic over at the /e/OS forum is your best bet. Really read through what they’ve done in the How to mount encrypted /data partition in recovery mode? - Fairphone - /e/OS community and then maybe ask questions about which details you need help with over there.