Why is the shutter speed for sports mode really slow on my nikon d5100?

My camera used to take really good pictures on action mode, but now the are all blurry. Any help?

Why not try it in manual mode? You can then select your iso. 800 or higher. That will then enable you to select a fast shutter speed, and aperture of your choice.Fast shutter speed = no blurred images of moving objects.

This would be easier to answer IF you'd taken the time to tell us whether you're shooting indoors or outdoors or if the event is at night outdoors.

If you're shooting indoors or at night and using the 18-55mm f3.5-5.6 zoom that came with your camera or one of Nikon's equally slow zooms like the 55-200mm f4-5.6 or 18-200mm f3.5-5.6 then your only option (other than buying the really expensive 70-200mm f2.8 zoom) is to use an ISO high enough to give you a shutter speed fast enough to keep motion from blurring. Depending on the lighting this may be ISO 1600 or 3200 or even 6400.

Here are two sites that I think you need to spend time on:

http://www.nikondigitutor.com/eng/d5100/index.html This video tutorial by Nikon will help you learn more about your D5100. Who better than Nikon to teach you about your Nikon D5100?

http://www.digital-photography-school.com/learning-exposure-in-digital-photography Learning about the Exposure Triangle - Aperture, Shutter Speed, ISO - is something everyone interested in photography should learn. Also, actually learning how to use your camera in Manual will end your dependence on those silly "Scene" modes.

In the future, when asking a question, please, please, please include as much information as possible. Conditions you were shooting in when you had a problem. The lens or lenses used. Anything that can help us help you.

Because you are sooting in auto mode you are at the mercy of the camera.

For sports action select shutter priority mode at 1/200th or so
adjust the ISO until the exposure falls in the aperture range of your lens.