Well all my friends had the same problem and what they all did yo fix it is:
Go to search on the desktop / start button and type in gameinfo.txt
It should find a few .txt all with the same name but there for each game that you own e.g. half life 1, CS etc
So look at the place that they are at and if it has half life 2 or css depending on what game it says missing gameinfo.txt, copy the right one into the bin directory of the game that is missing it.
I should then work and when you compile it will say, cannot find gameinfo.txt, looking in BIN directory.
It then finds it and presto you can map again