Taphephobia: Fear of being buried alive.
A phobia is an unreasonable sort of fear that can cause avoidance and panic. Phobias are a relatively common type of anxiety disorder. Phobias can be treated with cognitive behavioral therapy using exposure and fear reduction techniques. In many cases, anti-anxiety or anti-depressant medication proves helpful, especially during the early stages of therapy.
The word "taphephobia" comes from the Greek "taphos" meaning "grave" + "phobia" from the Greek "phobos" meaning "fear" = literally, fear of the grave, or fear of being put in the grave while still alive.