Something that's obstructed is blocked or closed off. If a spelunker — or cave explorer — finds that a tunnel is obstructed, she'll turn around and crawl back the way she came.
If a passageway is obstructed, you can't fit through, and if your throat is obstructed, there's a piece of food stuck there momentarily. The adjective obstructed shows up a lot in medical terminology, especially when the digestive or circulatory systems are unhealthily obstructed, but you can use it to simply mean "blocked." The Latin root is obstructionem, "a barrier or a building up."