I used to be able to see how long time ago someone was on Facebook. She uses the desktop website, not a smartphone app. For the last couple of weeks I can't see that. Do you know why? She didn't unfriend me. I can send her messages and find her in a search bar, so I don't think she blocked me. But why I can't see when she is on Facebook or how long ago she was on Facebook?
Probably she has changed her FB password and choose to logoff from all devices.
By doing this anyone can hide last login time. Make sure you have not logged-in on phone with FB or messenger.