Is it stuck or does it scroll but doesn't register? Is it a smooth scrolling wheel or one of the clicky ones?How do you fix a non scrolling mouse wheel?
Either way, open it up and clean out the wheel section. Lots of mice does their scrolling input with an optical sensor that trips when the spokes of the wheel is passing infront of it. If there's crud or even an eyelash down there it could block the sensor.