How come a Vivitar zoom lens isn't working with Nikon D5100?

My friend let me borrow a Vivitar 70-210mm zoom lens. It was working great and taking pictures then it just stopped working. My camera, Nikon D5100, says it's not attached. All help appreciated, thanks

One possibility could be that you previously had it in M mode and switched it to another mode that requires the electronics that this lens lacks.

Do you know for a fact that the camera and lens are "talking". Do the connections between lens and camera match exactly? Is this lens supposed to work on this camera? If you can't answer yes to all those questions, and perhaps more, there's your answer.

You need to be more clear when you say that the lens 'Isn't working'. What exactly is happening?

This is an older manual focus/aperture lens. It's a beautiful lens by the way. Anyway, there are no electronics on this lens so it does not couple with the camera whatsoever. You will need to manually focus it, set the aperture on the lens, meter manually and have some degree of photography skill to use it effectively.

The fact that you are confused about the camera not knowing it's attached says to me that you are an amateur who is confused and who needs some assistance… I'm happy to help, but I need more details from you.