At an impasse; at a point where no more progress is possible. The phrase can be literal or figurative.

What does it mean when someone is a dead end?

a situation that has no hope of making progress: Negotiators have reached a dead end in their attempts to find a peaceful solution.

How do you use dead end in a sentence?

1. Remember that adversity is not a dead end but a detour to a better outcome. 2. His research was at a dead end.