In the Steam games list, select the game and click the Properties button. Then click the Set Launch Options button. Add "-dxlevel 81" to the list of options that are there.
Launch HL2 and then just quit it. Remove "-dxlevel 81" from the launch options. That option is a one-time thing that should be permanent until you change it back. See if HL2 now works like it used to.
If that doesn't work, please use the Run Command dialog to run "regedit". Navigate to [HKEY_CURRENT_USER\Software\Wine\Direct3D] and make sure that the "useGLSL" value it set to "disabled".
Also, could you please report what model Mac you use and what GPU it has?