diff --git a/Library/Homebrew/brew.rb b/Library/Homebrew/brew.rb
index b514d00621..42911fa9a4 100644
--- a/Library/Homebrew/brew.rb
+++ b/Library/Homebrew/brew.rb
@@ -104,6 +104,13 @@ begin
     possible_tap = Tap.fetch(possible_tap.first) if possible_tap
 
     if !possible_tap || possible_tap.installed? || Tap.untapped_official_taps.include?(possible_tap.name)
+      blocked_tap = possible_tap && Tap.untapped_official_taps.include?(possible_tap.name)
+      if blocked_tap
+        [
+          "`brew #{cmd}` is unavailable because #{possible_tap.name} was manually untapped and cannot be retapped.",
+          "Run `brew tap #{possible_tap.name}` to reenable `brew #{cmd}`.",
+        ].each { |ln| onoe ln }
+      end
       # Check for cask explicitly because it's very common in old guides
       odie "`brew cask` is no longer a `brew` command. Use `brew <command> --cask` instead." if cmd == "cask"
       odie "Unknown command: #{cmd}"