a standstill or deadlock, some situation in which no progress, resolution, or escape is possible, often involving a disagreement between two or more parties;
e.g. Their discussions have reached an impasse since neither party is willing to compromise.
(noun)
a cul-de-sac, i.e. a path or road that cannot be crossed, traveled, or passed;
e.g. They reached an impasse in the road, which forced them to turn the car around and find a different route to their destination.