once you are certain that 1) warranty is over 2) it is not a software thing:
unscrew ALL the screws! and look inside.
From what I have seen there seem to be 2 common ways for the mousewheel turn sensor thingy:
1) Remember the old mouses with the ball that drove little wheels with holes? The holes were read by a photo sensor. Same mouses use the same thing for the scrollwheel. The sensor can become obscured by dirt so carefully clean it.
2) The scrollwheel rotates some funny shaped axle with cams that press on a switch. The axle can become too round or the switch can come lose or w/e. Most simple is to try to get the switch closer to the axle but that depends how it is build etc.
In the meantime page up&down keys can be used to zoom in Spring
