It sounds like the game got forced into a resolution that isn't supported or something along those lines. If you look in the steam folder for the game (under common in your steam folder, then Tidalis), you will see a RuntimeData folder. There will be a settings.dat and settings.bak file there, and those (intentionally) aren't reset when Steam refreshes all the game files. Those have your settings preferences, your resolution preferences, high scores, license key, and all that. If you delete them, or rename them to settings2.dat and settings2.bak, then things should revert to a "fresh" copy so that you could get it set to a resolution that works with your specific monitor.
Hope that helps! And let us know if you have any more trouble with that, of course,