Have you tried:
1. Do a FILE / NEW to create a new canvas. Did a canvas appear?
2. Make sure WINDOW / HIDE COMMAND PANELS is not selected. Once your sure it's not selected, try WINDOW / Reset All Window positions.
If that doesn't do anything for you, make sure you have only 1 version of SAI installed. If you had a trial version and then installed again, you may have two versions. Second, if your on Windows 7, make sure you are installed someplace 'safe' ... NOT any version of the program folder ... by default it should reside in a hidden folder in your user folder.
And if none of that works, write back with your OS, install path, and version of install, and we'll see if we can make it work.