When your neck is out of adjustment it can feel something awful. It might be difficult to play or impossible. There may seem to be some kind of slight problem. It just doesn't feel right, somethings off. Believe it or not sometimes it can actually feel fine except it buzzes just a little or even a lot. Perhaps a combination of any of these symptoms.
Unfortunately there's no one, universal spec for all necks. It's more like each neck requires a different set of rules depending on the available hardware on the guitar and the inherent disposition of the neck as well as the neck angle, as it aligns with the body. All of this may sound a bit confusing if you're not familiar with some of the terms, but fear not! There is a cure and it's a fairly simple one at that.
Since there's no possible way to cover all of these situations that may occur we'll start with the one that takes place in a perfect world. Let us presume that the neck is OK and the neck angle is correct and the saddle or other hardware is alright and is correctly adjusted. In this case it is simply a question of "RELIEF". Relief is a term that refers to distance between the bottom of a string and the top of a fret that takes place in the middle of your fingerboard. It's related to "ACTION" but not the same. To measure your neck's relief, hold down the low E string on the first fret with your index finger on your left hand. Now with you little finger on your right hand hold down the same string at your body fret (the first fret that falls over the body of your guitar). Next look in the center of those two positions and tap the top of the string with your right thumb in that center position down until it touches the top of the fret below and take note of the distance between the two. It should be 5-10 thousandths of an inch or roughly the size of your high E string or smaller. If it's a whole bunch more than that you'll need to tighten your "TRUSS ROD", an adjustable bar that runs the length of your neck directly below your fingerboard inside your neck. If there is less than 5/000 or no relief at all than you'll most likely need to loosen your truss rod.
I've included a chart below for your convenience. It shows where the adjustments take place on the different types of instruments. These adjustments are usually performed with an Allen wrench or a nut driver. If all this seems too much you could always refer this to a qualified guitar technician in your area.