My 55-200mm lens won't focus. It goes in and out, repeatedly without focusing. It used to work great. It will not work on auto, as it did before, or manual, or resetting the camera to default settings. When I switched lenses, the other one worked fine. It's just my Nikkor 55-200mm that won't work all of a sudden.

SOURCE: Nikon DX AF- S Nikkor 18-55 mm lens won't Auto focus

Is the switch on the side of the lens set to manual or auto? Sometimes it's the little things like this that we don't notice.

SOURCE: Nikon 18-55 zoom lens doesn't autofocus

You might like to check if the A/M slider switch on the lens by the Nikon label is set to A.
It sometimes gets moved accidently.
Set to M the lens will not autofocus.

SOURCE: Nikon D40x AF-S Nikkor 18-55 mm 1:3.5-5.6 GII ED Lense Trouble

I've found the same thing with my D40x and the 18-55mm AF-S lens. I took a really close look at it and found that the mount on the lens is badly worn on all three of the bayonet tangs and one is cracked at the base. What happens in my case is that the lens has movement when mounted on the camera - just enough to stop the contacts from mating properly. Hence, no photos. Am taking it to Nikon to get it fixed. I suggest if this is your problem, that you take it to the repair man.

SOURCE: Nikkor AF 80-200mm f/2.8D ED Lens, with broken auto/manual ring

Depending on where you live, you can take it to an authorized Nikon repair place or just send it back to Nikon.
In the US, they are in Melville, NY and ElSagundo, Ca.

SOURCE: Nikon DX AF- S Nikkor 18-55 mm lense locks

Try sending it to PhotoTech Repair Service in NYC. They are Nikon authorized, and can do the repair under warranty if you still have it. Also if you join they're facebook page, they will give you a 10% discount.
Here is the facebook page link, it has everything you need to send it in.
http://www.facebook.com/pages/New-York-NY/PhotoTech-Repair-Service-Inc/102527215194