Go to steam/steamApps/CS:S/cstrike , there is a text-file called gameinfo.txt, rename it ( for example gameinfoBACKUP.txt)
After that go to https://dl.dropboxusercontent.com/u/38118182/source%20sdk/cstrike/gameinfo.txt , right click save as, again put it in you cstrike folder and try to run hammer again.
If you want to play again you have to rename your gameinfoBACKUP.txt to gameinfo.txt.
Depending on what you want to do you have to rename the gameinfo every time, mabye there is a better solution...