Error when trying to load CSS

[KikiJiki]

My friend was trying to load steam a couple of mins ago and the box came up with Counter-Strike Source is loading then it came up with this box giving a warning does anyone know how to fix this?

http://img23.exs.cx/my.php?loc=img23&image=wa7qj.png

 

[SenorDingDong]

redownload CSS

 

[KikiJiki]

Ok thanks!