If it's an electrical failure, i.e. not making contact, there's a temp fix that applies to almost all GM starter solenoids. If you want to give it a try, here's how:
Take the old solenoid cap off, unscrew the positive lug completely and you'll see it has a square base. Push the copper lug out of the cap, rotate it 180 degrees and re-install in the new position. Also clean up the contact disc with some emery cloth. Turning the lug 180 puts the worn out surface on the outside where it doesn't matter and give a "fresh" unworn surface on the inside where it counts (that's the only place it actually makes contact). It's hard to explain without pictures, but if you take one apart, you'll see what I'm talking about. The lug is "ambidextrous" for lack of a better term.
Of course you should still order a new solenoid because this field expedient fix won't last all that long (it'll usually wear out again in a few months). But.. If you're ever stuck in the middle of nowhere with a dud solenoid, it's kind of a cool trick to know. Doesn't cost anything and takes about 5 minutes to perform with the starter out.