Good morning,
I am going to have to make a lot of assumptions here as i have no idea what you are asking. I am going to assume you are saying:
My main lens is not focussing however i tried another lens and that focusses just fine.
If that is the case, then buddy it's time to have a little poke around through the lens.
Before you go on the expedition through the internals of a camera lens. When you say it doesn't focus. Does it:
* Grind when attempting to focus (Gears problem)
* Just not even try (Software issue)
* Flash up an error (Camera connection issue)
* Somehow autofocus to infinity and get stuck there (Something else. I have this issue with a canon 18-55 and I have no clue why)
After you've figured out what it is, then go exploring. Make sure you keep track of what goes where because things could get a little disorganised if everything isn't kept intact.