There is a spring loaded lever on top of the mower deck that the cable runs to. Look underneath while you pull the lever, you'll see the spring and cable. It attaches to an arm that's probably gunked up. Work that arm back and forth to loosen it up.
It will probably help to really clean it up good so you can see what's happening with the cable and arm. It's not a hard repair, just work it loose again.
Brandon
April 2008