Games : The sand buggy has one wheel stuck in the ground and I can't continue

I'm playing Half Life 2 on The Orange Box. I'm in the "Highway 17" section, and at some point while driving along the sand, my buggy got one of it's wheels "stuck" in the ground. Here's a screenshot:

It got stuck somewhere shortly before the base where you have to shoot down the airship with the rpg. I literally used the gravity gun to push it along to that base. Now I can't get it from the soft ground onto the road to continue. Here's another screenshot:

When I try to push it onto the road, it does all kinds of flips and craziness, but that wheel never comes out of the ground.

None of my save files go back far enough before this bug happened (I feel stupid). What do I do?

Edit

I just tried walking back all the way to where I originally got the buggy, and that didn't help. I didn't really think it would, but it was worth a try.