TWRP is the recovery program which comes with Fairphone Open OS and since Android 7 also with Fairphone OS.
If you are curious, here’s a guide (different device, but TWRP works the same).
Normally you would get into recovery by keeping Volume + pressed while starting or rebooting the phone.
So a simple reason would be that this button was accidentally pressed or temporarily stuck.
That this message can be a false alarm by TWRP has been observed by users here occasionally.
I have no idea why it happens, sorry.
(A PC or notebook may occasionally not find its usual boot device for obscure reasons and cause similar concern before just booting normally after a reset or after turning off and on again … I don’t know how that works with smartphone boot procedures … but computer devices in general can be peculiar sometimes 
.)