If this is Half Life: Source, then you can start the game with this command line option in order to have it re-autodetect your screen and use the default size it first started with:
-autoconfig
This will not fix your config file, but it will ignore the current (bad) config files, allowing you to fix your resolution settings, close the game, then run the game without the -autoconfig
option to use your newly-chosen resolution.
You can add command line options to a game from within Steam by right-clicking the game in your Library, choosing Properties, clicking Set Launch Options, then pasting in -autoconfig
(or whatever command line option you're adding).
If this is Half Life (the original), then you can start the game using the -w
and -h
command line options to tell it to start up with a different resolution. For example:
hl.exe -w 800 -h 600
Will start the game up in 800x600 (4:3 resolution), and will allow you to fix your graphics settings to that next time you start the game without the -w
and -h
options, it'll use your new resolution settings.
Reference source: Command Line Options article at the Valve Developer Community site (thanks to 3ventic)