Burnout is a reaction to stress, usually in the form of exhaustion (often emotional, but physical exhaustion is also common.) It can also show up as illness or a general bad attitude. More informally, burnout is a sign that something needs to change in your life. We generally think of burnout as being work related, but it can be related to our home or family life or to school if we are also students. For instance, working full or part time while attempting to do as much around the house and for your family as a full-time stay at home wife and/or mother can cause burnout. Burnout is a particular problem for computing professionals because it is common to work extended hours and otherwise non-sustainable schedules. We often set ourselves up for burnout by starting a new project with an excess of enthusiasm and overwork and then later by attempting to fix problems with heroics. It is no coincidence that Extreme Programming (XP) includes working at a sustainable pace as one of its principles.
When you are in the midst of burnout, it is often very tempting to just quit your job and find a new career. This is probably a bad idea. The change you need might be a new career, but that is a decision that needs to be made with care and research, not simply out of frustration.
Do you have a part of your job that you particularly dislike? See if you can find a way to avoid doing it – perhaps by trading duties for a while with a coworker or finding a way to automate it. Don't put it off though, that will only give you one more thing to dread. If you do have to do that part of your job, reward yourself, even if it is as simple as trying to schedule it before your break.
Speaking of breaks, when you are burnt out, you need to take care of yourself. That includes eating well, exercise, getting enough rest, and taking your breaks. Almost every computer professional I know has the bad habit of limiting breaks to just long enough to hit the restroom, coffee or soda machine and have a cigarette if they smoke. Or they use their breaks to read their personal email. Try to do something refreshing or at least different – go outside and walk around the parking lot, play cards (actual cards, not computer solitaire), write a letter (an actual paper letter), sit in a comfortable chair and read a few pages in a novel – just get your mind and body doing something different for a little while.
While these things may help with the symptoms, you also need to deal with the root of the problem. If your burnout is due to the traditional IT project cycle and dependance on heroics, see if you can create a more sustainable pace either for yourself or, better yet, your entire organization. If it is due to factors beyond your control – for instance, being subject to micromanagement has been shown to result in burnout, your best choice may be to start a careful job search. If you realize that you are burning out because your current job or career path is something you find frustrating or overly stressful (for instance, I'm quite good at network management, but I tend to dream about my work exclusively and start to feel like I work 24/7 when I do it too often), then maybe looking into a different career or different area of IT is a good idea.
There are more good ideas for dealing with burnout in the article On the Job - Feeling Job Burnout? on the Work & Family Site.