Depending on year, that mower should have a pulley and belt drive. In short, the wheels are driven by a belt that rides a main pulley between the mower deck and the blades.
Remove and/or unplug the spark plug, and check out the belt below the deck. Is it there? Does it tighten when you press the lever to propell it? When tight, the belt should have .5"-.75" of travel when you push on it.
It may be a simple belt adjustment or replacement. It can also be a broken linkage, but doubtful.