It happened to me, and when I went to check on my game directory, I discovered that Steam had "verified" away my CivilizationV.exe

A bit later, I ran the Steam verify option and got a new CivilizationV.exe, this one was version 1.0.0.20. Guessing it must be connected to the patch.
So yeah, you might try seeing if your CivilizationV.exe or CivilizationV_DX11.exe got verified out of existence. To get it back, try exiting and reloading Steam, then right-click Civilization V, click Properties, click Local Files, click Verify Integrity of Game Cache. Good luck to you,
-- Joren