The question is pretty self explanatory: How can one make an Automator script to restore all hidden windows at the same time?
This AppleScript should restore all hidden windows.
The first action unhides any hidden programs. The second action restarts the Dock, which has the effect of un-minimizing any windows minimized to the Dock.
If you particularly want an Automator workflow, give it one action: Run AppleScript, and assign this AppleScript.