You have limited security clearance and no real idea of just how dangerous your job has become, until the morning you are sent alone into the Test Chamber to analyze of a strange crystalline specimen. A routine analysis, they tell you. Until something goes wrong. The next thing you know, the entire Black Mesa Facility is a nightmare zone, with sirens wailing and scientists fleeing in terror from the things their co-workers have become. Hordes of creatures from the far side of the portal are pouring through rifts in the local fabric of reality.
Monsters are everywhere.
You head for the surface, but the usual routes are impassable-closed off by the disaster, infested with headcrabs and houndeyes and increasingly larger and hungrier creatures. When even your own species turns against you, maybe you'll be glad to see another portal beckoning. But that's a pretty low priority compared to saving your own skin.