When you've just opened the Credit Roll window, before having touched ANYTHING else, try this ... Alt-spacebar, M, then use the arrow keys to move it back in to view.

If that doesn't work then you can do a Vegas reset by holding down the Shift + Ctrl keys while starting it. Beware though that this resets everything back to the initial default values, so any custom changes setups you've done with have to be done all over again. At least your custom templates should be preserved.