If new, be aware that you can return the phone within 14 days.
If new or not older than 2 years, be aware of the warranty.
https://forum.fairphone.com/t/power-off-reboots-phone-oups/43699/42?u=anotherelk
Else …
Else … contactsupport .
If it is clear that the OS has a problem, you could first do a factory reset …
https://support.fairphone.com/hc/en-us/articles/115001610926
… which should help with trouble brought in by user data or Apps, the factory reset deletes those and leaves only the OS as is.
If that doesn’t help, the OS can be freshly installed, too …
You can.
The Fairphone 2 port is at 15.1 currently, based on Android 8.1. There’s a fork with included microG, too.
Currently, there is no working multiboot solution for the Fairphone 2.
If interested, keep an eye on EFIDroid …
Else … you can always install any other OS again … oslist .