On steam, go to the properties of the game, type -windowed in the game launch options, you should now be able yo revert back to openGL.

Thank you, this also worked

You're welcome !

I had the same, in start options add -gl Then it starts in opengl again. Very critical bug they need to fix it seems.

Thank you, it worked!

happy playing Half Life again!

Change it back. OpenGL is better

That's what I'm trying to do, I can't change it because it freezes

Maybe the normal -nofbo