Blazer Won't Start

I have a 2000 Blazer ZR-2. With its age, and how thoroughly the last owners beat it up, it has some...special qualities. One of them being that the ignition doesn't automatically pop into the On position from Start. Not a big deal. It's a habit now for me to turn it back a click once the engine rolls over.
A coworker left his lights on tonight and borrowed my keys to use my Blazer to jump his car. I forgot to leave the ignition tidbit out, as I was busy with a customer at the time. He presumably left it in Start as he pulled it out of the parking spot I had it in and backed it in next to his vehicle, because it screeched at one point. Got his car jumped, yada yada. Gave me my keys back, things were fine.
Got into it after closing up the store and...nothing. Dash lights come on, radio kicks right in, I can hear the hum of the fuel pump but nothing else. No clicks, no attempts at cranking. I looked into maybe it being a stupid security issue, but the Security light doesn't flash when I turned the key. It shut off like it should.
I imagine it could be several things. Starter, solenoid, ignition switch? Not sure if him keeping the key turned all the way to the Start position the entire time he moved it and jumped his car gives someone a more concrete answer? I imagine I'll have to end up taking it to a repair place regardless, but it'd be nice to know what I might be in for.

Side note - The only troubles I've had engine-wise since I've gotten it is it bogs down a bit when it's been rainy. Once it warms up and I've driven a mile or so, it's fine. I'm guessing that's more of a distributor cap/rotor deal, but it doesn't happen frequently enough for me to be too worried about it, unless it could also be the cause of the current non-running issue.