It sounds like the snapshot manager has somehow been moved off-screen. Open the Terminal application (located in Applications > Utilities) while Parallels is not open and copy and paste this into it:
Code:
defaults write com.parallels.Parallels Parallels.MainApp.SSManagerXPos 0;defaults write com.parallels.Parallels Parallels.MainApp.SSManagerYPos 22
Then hit return and your snapshot manager window position should be reset and the window should be visible the next time you open Parallels and launch the snapshot manager.