Did you try a new config and now you don't like the settings? Just want to go back to "stock" settings?

1. Quit all of Steam (this is important)

2. Go into:
C:\Program Files\Steam\steamapps\YOUR STEAM NAME HERE\counter-strike source\cstrike\cfg

or (64-bit):
C:\Program Files (x86)\Steam\steamapps\YOUR STEAM NAME HERE\counter-strike source\cstrike\cfg

3. Rename config.cfg to old.cfg

4. Restart Steam and CSS