Why can't my tamron lens auto focus?

Help! My tamron Lens will AF (AUTO FOCUS) fine when I zoom in using the viewfinder, but not Live View does any one know what the problem might be? My camera is nikon D5100
lens 18-200mm

Added (1). I forgot to say it was working fine before

Hm.

I'm old school and never use Live View unless my camera is attached to my telescope.

You may have to see if Tamron is compatible with the live view auto-focus system on your camera.

If this was a Nikkor AF-S lens, there could be an easy way to troubleshoot this problem, but third party lenses, you are going to have to call Tamron's help line

Do a settings reset on your camera. You must have changed a custom setting.
Also, don;t use live view. It's slow and the screen is a major drain on your battery.