To be more precise, the keyboard shortcut "W" is to toggle between Preview and the default view mode. It's for the five previews shown at the very bottom of the Tools panel. Its found under a different part of that huge list of shortcuts as shown below:
There is no default shortcut for Preview mode under View > Screen Modes > Preview or any of the other modes listed there.
If you see the shortcut W listed where mine is, and if that's not switching to Preview, you might try restoring your preferences.