To fix open up your killingfloor.ini file (located in steam/steamapps/common/killingfloor/system)
find
[Engine.Engine]
RenderDevice=D3D9Drv.D3D9RenderDevice
;RenderDevice=D3DDrv.D3DRenderDevice
and change it to read as
[Engine.Engine]
;RenderDevice=D3D9Drv.D3D9RenderDevice
RenderDevice=D3DDrv.D3DRenderDevice
Save the .ini and relaunch the game.
find
[Engine.Engine]
RenderDevice=D3D9Drv.D3D9RenderDevice
;RenderDevice=D3DDrv.D3DRenderDevice
and change it to read as
[Engine.Engine]
;RenderDevice=D3D9Drv.D3D9RenderDevice
RenderDevice=D3DDrv.D3DRenderDevice
Save the .ini and relaunch the game.