What is the difference between dead and dormant?

Dead means dead. Something dead does not return to life.

Dormant means sleeping. Something dormant is waiting for the right circumstances to show itself again.

A dead volcano will never again spew lava, but a dormant volcano will burst forth when its pressure builds.

The high plains desert in Arizona may appear dead, but it is only dormant; for a brief moment each March it returns to life only to again fade before the burning sun.