What happens if a lens does not autofocus with Nikon D3100?

Is it hard (if so, how do you do it?) to try to focus it yourself or are you not able to use it with the camera?

If a lens doesn't have a built-in focus motor, you can still focus it manually, and you will still have auto-exposure with any lens from the past 40 years. It does take a bit of practice, being able to focus quickly and accurately, but it can easily be done. In addition, your camera will probably provide an indicator in the viewfinder to show when the subject is in focus, which makes it a bit easier.

Most new Nikon lens have a switch on the side which allows you to manually focus the lens. It's not hard to do at all. You just twist the lens barrel until what you want to shoot comes into focus. That's the only choice we had before auto-focus was invented.
Yes, you can do this with the D3100. Just look on the left side of the lens.

For generations (and I've lived through a couple of them), photographers have manually focused their lenses to capture the memorable images of our time. Any photos taken before the early 1980's and many after that, were taken with manually focused cameras. That includes every shot in National Geographic, Sports Illustrated, and in every daily newspaper in the world.

Older dslr's had special aids in their focusing screens but your D3100 is still capable of good manual focusing if you learn the techniques.

http://www.dummies.com/how-to/content/how-to-focus-a-nikon-dslr-manually.html
http://www.pistonheads.com/gassing/topic.asp?t=1136211

The DSLR's usually don't have the split ring and microprism optical aids to focusing that the old film SLR's have. So it is harder to get manual focus as accurate. There's usually some kind of electronic assist to tell you when you have it right or very close. It is rarely going to be quite as accurate as a functioning auto focus, but if you stop down a little you probably have enough depth of field to cover for it.

Back in the day, all cameras manually focused.

True, these days there's no ground glass element to assist in focusing, but there's a focus indicator on Nikon cameras that tells you when you are in focus.

And since it is the same focusing electronics that the camera itself uses to autofocus, it should be as accurate.