My preview panel not showing selected thumbnail

It's always a good idea to provide your OS (and what release) as well as which release of Bridge you are using. Does this happen for all folders or just one? What kind of file is not showing up up in the Preview Panel? Do any other files show up from this folder?

Nonetheless, I suggest you Purge the Cache for that folder (Tools menu). And, if that doesn't work, have you tried to reset the Preferences.

Quit Bridge if Open. Just before clicking on Bridge in the Dock or double-clicking the application for the Mac, press Command-Option-Shift. For PCs, press Control-Alt-Shift. Then open Bridge, BUT KEEP your fingers on those keys until you see a window pop up. From the window, the options are: “Reset Preferences" and "Purge All Local Cache Files.” In your case, check both of them.