Yes, this is my fourth reply on trackballs tonight. If you have some rubbing alcohol, dab it on your finger and scroll the trackball around. DO NOT use a q-tip or cotton swab. If you don't have RA, lick your finger instead...seriously, it's a quick-fix.
This is only if the trackball is gummed up. If it's broken, you'll have to replace it...obviously. If so, check out this web site for replacement parts.