Having had an issue with my Apple TV, I have now managed to fix it by the simple expedient of unplugging it from the power lead and plugging it back in.
I did try logging out and back in again, this didn’t work.
I am slightly annoyed that the solution wasn’t available from the Apple TV menu, a restart option, or even the option to turn the device off (and not just sleep). This means people could be resetting their Apple TV, when all they actually need to do is turn it off and back on again.