My CSS broke somehow

[Mallissin]

Fairly common error.

Try running down Valve's FAQ for it.

https://support.steampowered.com/kb_...5204-UOSN-9830

It seems more common in newer video cards (last 3 years) because of anti-GPU crashing measures in the drivers.

This is what happens; you load the game fine but when the game starts to load a map (a tougher job than just loading the game) it's causing the GPU kernel to crash for some reason, forcing the driver to try to reconnect and leaving the game without any video cards available to render the map on, which causes the game to crash with that error.

There could be many causes, from a bad setting in the game's video settings or your video card driver, to a corrupt model or texture in your CS:S directory.

So, before doing #4 on Valve's walkthrough, do a search in your Steam directory for any files modified or created in the last few weeks, move a few out to a safe place, try loading the game and repeat until the error goes away. Eventually you'll find the culprit if the problem is indeed a corrupt model or texture, or come to realize a complete reinstall is in order.