Flags suddenly not working

[Nevermore0]

I have flagged photos but when I select the "Flagged" filter, nothing happens. All of the photos are still displayed, whether it's in the library or develop view. It was working one minute and then I imported some of my Photoshop edits (which I've done a million times before) and it stopped working. Maybe I pressed a key that's making this not work?

[AxelMatt]

In a first step I would try to reset the Lightroom preferences to the default settings.

https://www.lightroomqueen.com/how-do-i-reset-lightrooms-preferences/

If this doesn't help we need more informations about your environment. 

• Which operating system do you use? 
• Which version of Lightroom do you use? 

Please post the exact version and not only phrases as "recent", "latest" and so on. 

What has been changed on your system before the issue occur?