DSLR not focusing/all blurry?

So recently I got a Nikon D3100 DSLR.
It was focusing fine and everything until one day where everything suddenly got really blurry no matter what I did. If I zoomed or unzoomed, everything got blurry. If I stood far away, it was still blurry. I didn't really mess with it other than turn the dial to change its modes (i.e. Auto/Manual/Portrait/Sport/etc.)

How can I get it to focus and sharpen pictures again?

Added (1). Also, while normally you can get pics where the subject is sharpened but the background is blurred out, now it's like the opposite.

My friend set it to Manual when she used it a day before this happened. (Maybe this is why? I'm just not sure)

Before you panic, look at the right side of the eyepiece of the camera. There you'll find a small dial designed to adjust for an individual's eyesight. Often, that dial gets moved by accident, causing "everything to get blurry."

See if that works.

Of course, also check if the autofocus switch on the lens is set for A (auto) and not M (manual).

Put your dial on auto and see if that helps

Did your friend set your camera's exposure system or lens to manual?

Your lens has a M/A switch. In the M position, it is manual focus. In the A position, it is in autofocus. Make sure the switch is in the A position.

Switch to AF (away from M - manual) to get autofocus to work again.