Can I repair a broken LEGO brick with glue?
LEGO is very good about replacing broken parts - for free even.  Visit the web-site and look on the bottom of the page under Customer Service for a "broken or missing parts" link (I can't check at current locale).

This is worth trying even if the set is old or you didn't acquire it brand new. LEGO doesn't usually care where you got the set; but if it's not recent there's a risk that the part is not produced anymore, and then they won't be able to replace it. But try anyway, they're usually very friendly.

You can also try calling Customer Service directly.