Welcome to the community forum.
It has been observed that this can be a false alarm by TWRP.
The OS still might have been there, a possible update hickup could still have been solvable at this point.
Only tried? Or really did?
If you really did a factory reset, your data on the phone would now be gone, and you could try to install the phone from scratch, see here … 🇬🇧 🇩🇪 ✏ Installing the Fairphone 2 from scratch in case of failure / Komplette Neuinstallation des Fairphone 2 im Fehlerfall.
If you have no backup of your important data, and if you are not sure about the factory reset, we should better try to get https://support.fairphone.com/hc/en-us/articles/207914363 working, since it supposedly doesn’t delete user data.
Since there can be different issues which would give different error messages, it would help if you could post the complete command line/terminal commands and their output, or screenshots.