First off, you didn't post anything about your year/make/model of your Jeep, so this is mainly generic information.
Your passenger compartment heat working relies on engine coolant in your heater core. Since you started by saying your heat was only 'kind of' working, that's a big indication you've been driving for a while with low coolant. (Water is much better at transferring heat than air.)
How bad is it? Is everything going to be ok? Dunno. Since it sounds like you may not be ready to tackle tearing your engine down to check things, I would recommend bringing it to a shop to have a compression test done. If things cooled without cracking, warping, or melting, then you may not be hosed. I'd certainly recommend checking and filling/replacing engine fluids (oil, anti-freeze again) to flush out anything nasty that accumulated in there. Otherwise (since you've already added coolant back to the engine) if it's cracked, you'll see either water in your oil (it'll look milky/brown) or your coolant won't be pure (green/orange/whatever color it started as.)
It wouldn't hurt to call a local shop and see if they have a way to bring it in for you if you want to have it checked, otherwise, the damage is already done so to speak so why not drive it there and see what they say.
97 TJ 6cyl, 5speed 37" Krawlers on American Racing 17" wheels. (Beadlocks someday)
RE 5.5 Long arm, Currie 9" front and rear with ARB's, Warn hubs and axle shafts, CTM u-joints
Optima Dual Batteries, OR Fab roll cage, custom rock sliders, SMF shorty bumpers front and rear.
My Jeep Site: www.jeffsjeep.com