Solution req: Viewfinder in Nikon D5100?

Whenever I try to take pic with viewfinder, it focusses but didn't clicks? What could be the reason. When it is in live mode viewfinder is not active and vice versa. I use d5100 and my Lv is working perfectly but not with viewfinder. Please help me out. Some focus assist light is turning on and turning off but still not clicking.

You are trying to take a picture in the dark. Move to a well lighted area then try again.

Take note that what is low light to your eyes may as well be pitch-black darkness to the camera already. Learn to adjust. Photography is the act of recording light. If it's dark, what is there to capture?

When live view is on the viewfinder is inactive and vice versa. That is normal. Why would you want a bright LCD right beside your eye when you are looking through the viewfinder? And why would you want to waste battery power on the viewfinder when you are using live view?

Are you sure it is actually focusing and you aren't too close to the subject? Just because it stops whirring doesn't necessarily mean you have successfully focused. Has the green circle for focus confirmation come on?