There are a couple of possibilities:
1. Sometimes saved games become glitched for no apparent reason. Restore from a previous save, and see if that one works. Since we don't know WHEN the saved game became glitched, you might have to go back a couple of saves to find one that isn't glitched or download a saved game from the Witcher Saved Game Bank
2. If you've been playing for awhile, the game may run out of memory. Reboot your computer, start up the game, and then try.
3. Try turning down your graphics settings in the Options panel.
4. Sometimes the first symptom of an impending hardware problem is strange performance in a game. Check your memory and graphics card and make sure they're in good working order.
5. A game file could have become glitched during installation. If nothing else works, try uninstalling and reinstalling the game.