Why does my camera go blurry in video mode?

My Nikon D3100 goes blurry in video mode when i zoom in, anyone know why? I'm gonna film my school show, what's the best options? It will be at night but there's a spotlight
my Lens are 18-55mm

There are special techniques and some accessories to make a dSLR into a usable video camera

One is a LCD loupe to assure sharp focus.

http://www.luminous-landscape.com/reviews/accessories/hoodloupe-3.shtml

And more on using a dSLR to shoot video.

http://www.luminous-landscape.com/reviews/camcorders/cam-cam.shtml

Shooting at night even with a spotlight, will require some special skills. For one you need to assure that the cameras white balance is set to incandescent to match the white balance of the spotlight. And make sure that you keep the camera aimed on the subject being lit by the spotlight.

Really you don't have the right gear to shoot a stage show. Your lens is too slow and the light may be so low that you will get a lot of noise in the video.

Do you focus… Manually I mean?