You may have slipped a disc if it's really that bad.
Try the following though:
1.) Lie on the floor face down with your arms out as though you were going to do a press-up
2.) Slowly pressup with your arms while keeping your groin FIRMLY on the floor so your back is arched.
3.) Hold the pose for a few seconds and then release.
4.) Repeat for a few minutes or until you get tired.
5.) If it's specifically on one side, try and move your groin slightly to one side (as in out of allignment…think of your body as the axles and BB of your bike. Your head is the front axle, your feet are the rear axle and your groin is the BB. When doing this normally all are in allignment….try raising or lowering the BB toward the side of the pain) and do the same process. It's a little harder and I wouldn't do this straight away.
Try not to sit at a desk for more than 30 mins as your muscles will knit.