Why isn't TTL BL working on my nikon camera SB600 flash?

Why doesn't my TTL BL show up on my nikon sb600 flash? It used to automatically change the zoom distance when i zoomed my camera in and out and it stopped doing that too, all it does is say M. I hit the MODE button and all it does is cycle through and doesn't change TTL. Any suggestions why its not communicating with my camera or sensing when i zoom in and out? The photos are turning out terrible and extremely overexposed so i know it can't be working right.

Read your manual.
beyond that clean the contacts on your camera and your speedlight. If the flash is firing it's just a matter of reading your manual and figuring out how to set it back to a fully automatic mode. Seeing how I'm pretty Nikon illiterate I can't tell you how. Someone else may hop in that can though!

Reset the flash. That's covered in the manual. The SB600 is easy to mess up with the plethora of settings!