I have experienced on several occasions, on various Mac’s, where the Dashboard functionality would cease to just work.
Couldn’t really figure out why and ran the usual Disk Diagnostic tools, DiskWarrior, Techtool’s and various other app’s.
Nothing as simple as toggling the Dash Board preferences either. After several trips to Macfixit to check out the forums there, I posted on the forum there about this issue. Generally I have had very good experience with the people on here, so many quick responses for the most part, so worth checking out.
The initial Macfixit link to all the various thread can be found here at 
Dashboard? Refuses to open and show it’s self.
But for the impatient of us, including me, here is the short solution to the various threads left on the forum.
< In Terminal enter (or copy & paste) the following : defaults write com.apple.dashboard mcx-disabled -boolean NO
< Hit Return
< In Terminal type : kiiall Dock
< Smash Return as hard as you can.
This should bring back the Dashboard to it former glory