Up inside the plug wire rubber end shroud is a circular bent piece of sheet metal that fits over the rounded head of the plug wire. It is just a piece of sheet metal bent almost into a circle, but with a small gap in the circle. Unless this is broken off, just reach up inside the rubber hood with a pair of needle nose pliers (or to use regular pliers peel the hood back carefully, like rolling a sock inside out) and crimp the metal loop slightly smaller (being careful not to totally flatten it - takes very little force), till it fits snugly over the spark plug. This is normally all it takes.
If the metal fitting is broken off - almost all spark plug wires have a separate 90 degree rubber housing at the end - it may look like a one-piece wire, but usually you can slide the 90 degree bent "hood" back on the wire, working the inside metal plug fitting out as you do so. Then you can pry off the old metal loop and put on a new on, if you can find one at a mower service or auto parts store.
You can find the complete parts list on the sears website, if you end up needing to replace the wire - involves removing the metal housing to replace the wire. Or if you take it out, an auto parts store can probably fix you with a replacement wire.