HL2 Wont Open

[G4B3]

Here is the error message that pops up: Error Message

It just stopped working, it had been working perfectly before.

My specs are:
Intel i7
Windows Vista Home Premium 64 bit
6GB of ram
500GB HD
ATI Radeon HD 3450

Help is appreciated! Thanks.

 

[AtomicSpark]

https://support.steampowered.com/kb_article.php?ref=7169-TIAV-5361