LR screwed up after changing graphics card?

Question

After successfully working with LR since version 4 over a long period with thousands of photos I now have a strange problem which I can´t find a solution for on my own.
What´s wrong?
-LR doesn´t show any images, neither thumbnails in import mode nor full images or thumbnails in development view
-There is only a grey area where the photo should be.
-Old photos or newly imported pics all have the same problem.
-switching between files causes loading progress with plausible duration, but the image does not appear
-Histogramm, Metadata etc. are shown correctly, but no image nowhere
-Photoshop/Camera RAW on the same computer with the very same files work without any problem
What did I try to fix it?
-renamed the catalog file and created a new one. No luck. Same effect while importing and after that in the development view only grey again...
-renamed the previews, no luck
-renamed the agprefs file to start with fresh settings which seemed to work because I had to set the user language and so on. But - no impact on the problem.
-searched the faq and several boards, discussions, Google. No answer. Only one user with the same problem, he also got no answer...
What I may have done to get into that kind of trouble:
-I changed my graphics card some time ago. I had a NVIDIA 9800 GT before and changed it to a NVIDIA 640GT.
-I took every OS update, NVIDIA driver update and Adobe CC/LR Update and did not use LR after a couple of such changes.
Some facts about my system:
-Win8, Core i7 920, 6GB RAM, SSD/HD with enough space
-LR 5.3 latest update
Please help. Thanks in advance.

Answer 1

I would first suspect the video driver.  Video drivers can cause weird problems, sometimes affecting only one program. 
Things I would do, many of which you may well already have done:
First off, set a system restore point, so you can recover what you do.
Check the video driver, and make sure it's the right one.
Try earlier versions of the driver.  The NVIDIA 640GT is not new, and may work better on older drivers.  My GTX 550 doesn't like the current drivers, and is significantly faster with earlier ones.  You can also go into Device Manager and roll back the driver to previous versions.  I don't update video drivers unless there's a reason: to fix a problem, say. New NVIDIA drivers are usually only to support new hardware. 
Try removing the driver (in Device Manager, use Uninstall for the video card).  That makes Windows default to a basic VGA driver, and then get Windows to update the driver (in device manager, update driver).  That will get what Microsoft reckon is the latest driver, which is not generally what NVIDIA think is latest, but may be more stable.
If you've still got the old card, try that (just to check whether it is a video card issue). 
Uninstall and reinstall LR.
I know that's the obvious stuff, which you've probably done. 
PS - as ssprengel says, check the monitor profile. Definitely the first thing to do!