I should still be in Surface Tension, so here’s the problem I found:
The section with the dropship and the alien grunts, I stayed in the tunnel and kept killing soldiers as they ran down into the tunnel. Eventually, as I made mistakes and died, my save files stopped loading and I had to force quit Black Mesa in order to exit the frozen loading screen. I think that, because the dropship seems to generate an infinite number of soldiers, the amount of bodies eventually corrupted the save files.
My fix: I simply rushed out, shot the dropship down with rockets and proceeded as normal.