Problem with my Nikon D5000?

I bought a Nikon D5000 + Sigma 70-300mm DG Macro (no OS no APO)

but now, the I can't autofocus. When I half-press the shutter, the focus ring would go all the way out, and never return. But, if I put the lens cap on, it works well, the focus ring would go out and back again.

I think the problem started when I resetted the settings.

Added (1). @Pat,
I already tried that. I always make sure to explore before asking questions.

@FhotoAce,
I bought the camera plus lens used, so I can't really return it. I just hope that if there's any defect, it would be on the lens and not on the camera body.

I bought the camera last January 17. And here are my shots last January 22-23.
http://www.facebook.com/...3396667125

Added (2). @Rauf,
The Sigma has a motor. I already used the lens for two weeks. Click the link above to see the pictures. And, besides the lens was already used since November 2009. Sigma and Tamron added motors for lenses that will be mounted on a Nikon.

Strange. Try changing the setting to Normal mode ( the mode that is just basic shooting.forgetting what its called haha ) And reset it. If that doesn't work then i have no idea. Maybe youll lense is broken. If you just bought it i don't see a problem with them giving u a new one that works

I just talked to a colleague and he was complaining about the the same lens on his Canon 7D. He said it cost $120 to have it repaired plus shipping.

Have you called Sigma?

Easy. Sigma 70-300 DG (non-APO) doesn't have a built in motor. The D5000 only supports all functions on AF-S and AF-I-type lenses. For the rest, there are varying degrees of compatibility but, suffice to say, if your lens doesn't have a built-in AF motor, you'll be limited to manual focusing.