Have you tried resetting all Photoshop tools and if that does not fix try resetting you Photoshop preferences
How do you reset Photoshop tools and preferences?
I have relaunched Photoshop while holding down Ctrl Alt Shift, but this made no difference - image still appears as thumbnail which can't be dragged or enlarged
Did you get the dialog that ask if you want to delete your Preferences. If you did not get that prompt and reply yes the you did not reset your preferences. You need to start Photoshop then immediately press and hold Ctrl+Alt+Shift if done quickly in time you shot get that dialog to delete your preferences.
Has anyone figured out this problem? I'm experiencing the same thing over here....
Did you try resetting your CS6 Preferences?