Auto focus messed up my manual focus?

Okay so i set my lens to autofocus to see how well it worked and i took a picture up close. But when i switched it back to manual, it won't focus on things that are far away, i can only focus on things that are close up. And when i used the autofocus to shoot something far away and put it on manual to shoot something close, i couldnt get it to focus. How could i fix this? I usually only use manual but after autofocus messed everything up i can't really use the manual setting anymore. I have a nikon d3100

I would say that unless something has happened inside the lens you're not using the lens properly.
If there's something wrong inside Nikon will be able to tell you that. If the lens is under warranty they will fix it free. If not it's your dime.

I'd suggest you try using the lens on auto focus all the time - at least for a while. Try all kinds of shots. If you discover that regardless of what you do the lens won't focus call Nikon for advice.

We're assuming you're talking about the 18-55mm kit lens, right?

If you've moved the autofocus switch on the lens from A (auto) to M (manual) it should work correctly. Does the focus ring rotate, or is it locked? If it's locked, then the problem may be in the switch. I'd also look to see if the gold contacts for the lens and body need to be cleaned. Just gently wipe them with a clean, lint-free cloth and see if that makes a difference. The camera's focusing sensors may not be communicating with the autofocus motor in the lens.

The other suggestion would be to take your camera and lens into a camera shop. Try a different lens on your D3100 and try your lens on a different body. That way, you'll find where the problem lies.