A problem may occur in OS X where, when specific programs are launched, they may be placed outside the bounds of the current desktop area, resulting in the inability to interact with the program. If this happens, the program may be seen in Mission Control and Activity Monitor, but when Mission Control is deactivated the program slides off to the side of the display to again be hidden.
If this happens to your system, it could be because of a faulty window setting that occurred when using multiple displays with your system. So, as a first attempt to fix it you might try toggling different display modes. To do this, first try changing the resolution of your monitor, and also attach a secondary display to your system and try different resolutions and modes such as mirroring and extended desktop (set in the Displays system preferences).
When performing these steps, try quitting and relaunching the program to see if this triggers usable window placement settings.
Next, try switching to the program either by clicking its Dock icon or by using Command-Tab, then choose a window adjustment setting from the Window menu. You can try using the "Zoom" feature to resize the window, or you can hold the Option key and choose the "Arrange in Front" feature, among others, to try gathering the windows on the main display.
Finally, if these solutions don't work you can try using a script to force the arrangement of windows on the main display. One such script is available here, and can be run by opening the downloaded script file in the AppleScript Editor utility, then simply clicking the green "Run" button in the script editor's toolbar.