The player has reached a Dead End when no actions whatsoever can carry the game forth, and the game progess is helplessly halted.
Dead-ends are generally impopular, since the game can be unnecessarily difficult to win, and any attempts to solve it will be a waste of time. Dead-ends occured primarilly in Sierra-games (several of the Police Quest games can go on long after a specific puzzle, but are yet impossible to beat if the puzzle in question is not solved properly), and are very rare in e.g. LucasArts games, if they exist at all.
This is generally considered a case of bad game design, especially if it takes a long time for the situation to be realised by the player.