The difference between OpenGL and Direct3D9.
▸ OpenGL allows the game (such as the chat, actions and everyhing else in the game) to continue running wilst you are tabbed out. For example, I have two Monitors, that = two screens. Because I'm using OpenGL for FeralHeart currently, on my left screen I can see FeralHeart's chat continuing to move, the stars are flikering and basically everything is running in sight whilst the window is not in use and I am typing here.
▸ Direct3D9 does the opposite. Driect3D9 pretty much 'pauses' or keeps everthing in the game 'still' whilst you are tabbed out of FeralHeart and doing something else. It freezes, basically, giving you better performances elsewhere.
Solution 1 - The only thing I can currently think of (without any screenshot provided to look at of what message is coming up), is that you're missing the 'ogre.cfg' file in your main FeralHeart folder. If that is the case, then you can download it here. If you download the 'FH Contents Download', you will find the 'ogre.cfg' file in there. Once you have found it, drag and drop it into the main FeralHeart folder and replace when it says so.
Solution 2 - If you're still having issues, go back to your FeralHeart folder and open the 'ogre.cfg' file in NotePad and replace everything in there with the following and save the changes after:
Render System=OpenGL Rendering Subsystem
[OpenGL Rendering Subsystem]
Colour Depth=32
Display Frequency=N/A
FSAA=0
Full Screen=No
RTT Preferred Mode=FBO
VSync=No
VSync Interval=1
Video Mode=1280 x 800
sRGB Gamma Conversion=No
[Direct3D9 Rendering Subsystem]
Allow NVPerfHUD=No
FSAA=None
Floating-point mode=Fastest
Full Screen=No
VSync=No
Video Mode=800 x 600 @ 32-bit colour
sRGB Gamma Conversion=No
I say this because you might have to start using OpenGL unless or until you get things back working again.
Solution 3 - If all fails, try doing what was suggested previously and attempt to update DirectX (which is the same as Direct3D9).
If you're still having issues, then maybe restart your computer for things to kick in after updating DirectX, or maybe you should consider updating your windows. To do this, go to the start menu and search 'Windows Update'. If nothing appears (very unlikely, but let's just be prepared), then direct yourself to Local Disk (c:) \\ Windows \\ system32 and run the execute named 'wuapp.exe' to update your current windows program.