Okay, so, first let me say.
CSS worked perfectly, just yesterday night I was playing it.
Today, I go to run it, it pops up and the screen flashes the background for the main menu, then blinks again and goes black on just my main monitor (I have Dual Monitors) with the circular loading cursor, and it pops up with the Windows Error Reporting saying "hl2.exe has stopped working"
I have taken a look at the event logs, and here's the info.
Faulting application name: hl2.exe, version: 0.0.0.0, time stamp: 0x4445c334
Faulting module name: scenefilecache.dll, version: 0.0.0.0, time stamp: 0x4474ce19
Exception code: 0xc0000005
Fault offset: 0x00003e2c
Faulting process id: 0xb00
Faulting application start time: 0x01ca724357f26880
Faulting application path: f:\steam\steamapps\axcolddecember\counter-strike source\hl2.exe
Faulting module path: f:\steam\steamapps\axcolddecember\counter-strike source\bin\scenefilecache.dll
Report Id: 9b2950f0-de36-11de-ace8-001fbc005108
Tried the obvious thing, a simple reboot, and also (something I've had issues with before on Vista causing a Engine Error) turning off the In-Game Friends interface.
I have also tried deleting "scenefilecache.dll" and it won't start without it. So, I go to "Verify Integrity of Game Cache" and it downloads that dll again, and it still will not start.
I'm not sure why this just randomly happened. I'm not sure if I might have gotten a new update from Microsoft, but I didn't notice Windows updating. Neither did I notice Steam, or CSS updating.
I tried updating DirectX, because I saw somewhere that that might help, but nothing changed.
BTW, Here are some specs (if it helps at all)
Windows 7 Ultimate 64-bit (OEM)
Running on a 4x120GB SATA2 7,200RPM FakeRaid array (onboard the mobo)
EVGA 780i SLi FTW
Intel Core 2 Quad Q9550
4GB Mushkin DDR2-1066 @ 5-5-5-15
PNY Nvidia GTX285 1GB GDDR3
I don't think this would make a difference, but I have the 4 disk raid array partitioned into 4 partitions. Windows7, Windows Vista, Applications, Games.