Do you use steam? My guess is you do.
Are you securely connected to the internet?
Are you on the "Off-line" mode of steam?
Also there is a settings tab that the bottom of the steam menu (when searching for servers) though i don't think it says "settings" if you click on it you can choose what it is you want to see come up. (i.e. passworded servers, any server that isn't full, any server that isn't passworded etc.)
if this is the case, you have 3 options. Either
a) contact Valve, showing them your original Half Life / CS cd and the cd key
b) go purcahse a real copy of HL
c) you already registered that cd key a long time ago and have forgotten