Why won't my Nikon D3100 autofocus anymore?

I got my camera about 3 weeks ago. It was new and it was working fine until about a week ago while i was on vacation. I used the lens 18-55 (which was the one that it came with).It used to autofocus perfectly but now it won't. When I go take a picture it takes the pic within 1 second like just one click and it takes the pic. It doesn't focus, nothing. The only way I can get the pics to focus is if I twist the front part of the lens. AND yes I read the manual and YES I know that it will focus with if you press halfway on the shutter button. Mine still doesn't work even if i press halfway. Can anyone help?

Added (1). Uh maybe i'm not sure. As you can proabably tell i'm not really good with this camera. Do you know how to fix it if i did? Cause i guess i was playing around with the camera and maybe changed something i wasn't supposed too.

Added (2). @AWboater Thank you So MUCHHHH! Haha i feel so stupid for not knowing that haha thanks again

Not to insult your intelligence, but did you happen to flick the switch to manual by accident?

Your lens has a M/A-M switch on the side of it. This switch puts the lens into AUTO/MANUAL or MANUAL focus mode. Make sure the switch is still in the M/A position.