I'm in the middle of a game where I have 4 heroes (1 summoned, 3 hired), no problem. I just summoned a champion and he had no skills at all. I've since summoned a hero and the Chosen, and they're all the same. So I've seen the swordsman and knight with no skills, chaos warrior with armor piercing and nothing else (no casting ability, but can still use a ranged attack), chosen with just magic/missile immunity, and the sage with nothing.

Is there any way to fix this, or is the game just hosed? I've tried reloading and restarting with no luck.

Huh, never seen anything like that myself. Are you using any modifications? Perhaps there is a way to correct things with an editor back to a more normal state, but in my (albeit limited) experience that seems to mess things up in one way or another.

I'm using the Insecticide patch, but I've played probably 8 or 10 games with it with no problems. I also just noticed that my existing heroes have also lost all of their skills (not good to get into combat and suddenly discover that none of your caster heroes are casters any more).