For users like myself who use multiple monitors, some versions of Parallels Desktop will develop an issue with the Snapshot Manager. When you click the button, like a regular modal dialog the rest of the interface will cease responding, but nothing will appear onscreen. Normally the only options for clearing the dialog are to press the Escape key, or force Parallels to quit. In most cases, reinstalling Parallels won't even fix this problem. After some intensive forum searching I came across a solution. Open Terminal and type the following commands:

defaults write com.parallels.Parallels Parallels.MainApp.SSManagerXPos 50
defaults write com.parallels.Parallels Parallels.MainApp.SSManagerYPos 50

You'll be able to open the Snapshot Manager right away; there's no need to restart Parallels.