Ha, forest....
Anyways, like people have mentioned there's a ton of potential reasons, and the only way to figure out is to see the recipe and gravities. Assuming the recipe was constructed properly (either from a kit or your own recipe), it sounds like a case of excess sugars. There are several things that could lead to them, but probably the most common ones are either the use of extract that contains less fermentable sugars than was expected, or an insufficient amount of yeast used. If it was an extract recipe, its possible that some of the extract got caramelized, leading to sugars that the yeast weren't able to ferment. Also, if you didn't use enough yeast (either an old batch of liquid, no starter for liquid, or non-rehydrated dry yeast), its possible that you underpitched and the yeast quit before they were supposed to.
Of course, without a recipe and gravities this is all academic. It should be noted that both of the above problems would lead to higher gravities as well, so those could be especially important to have handy.