Fess up to your need for human contact. Just because you're going stir crazy doesn't mean that you made the wrong decision when you decided to work from home - just that you need to make some changes.
Make lunch plans with friends or colleagues who work in your profession. If possible, arrange to meet with them the same day every week for a little socializing and networking.
Pick up the phone and call some of your colleagues, especially if you have any who also work at home. They may be in the same boat.
Send e-mail to friends and colleagues; it's the cost-free way to keep in touch, even with people halfway across the globe. Just don't forget to make some plans to meet people in person, as well.
Join a professional organization, then get out and attend any classes, lectures or social events the organization sponsors.
Consider working in an office environment one day a week. If you're a high-tech-translator telecommuter, arrange with your boss to work at the office occasionally. If you are a freelancer, see if any of your clients would like you to work at their offices some days.
Get out of the house, even if only for a walk around the neighborhood or a trip to the store. Don't let yourself fall into the trap of spending all day, every day, inside your home.
If the above aren't enough to keep you feeling plugged in, consider forming a business partnership with one or more colleagues; perhaps sharing your goals and interests with someone else will provide the stimulation you need.