Welcome to the community forum.
Would have been nice to know which OS version you are coming from, and the error message. Anyway, here’s something you can try …
Keep the power button and Volume + pressed for about 15 seconds until the phone reboots, keep Volume + pressed further while it reboots until you get into the recovery mode, Team Win Recovery Project (or TWRP for short) should appear.
Worth a try when in TWRP …
A little more detail in between …
If this doesn’t work, download the “Manual installation” file for Fairphone Open OS from here (since you tagged openos) to a computer …
https://code.fairphone.com/projects/fp-osos/user/fairphone-open-source-os-downloads.html
… and follow the following procedure with it (instead of using the ZIP archive offered there) …