When you reinstate someone, you return that person to a position or job. A principal might reinstate a laid off teacher when the school starts hiring again.
Bosses can reinstate employees, countries can reinstate kings and queens, and governments can reinstate, or bring back, old laws. For example, during times of war the United States could very well reinstate the draft, which is not currently a law but has been in the past. Although it's not as common as reinstate, the word instate, from which it comes, means "to put someone in a certain state or condition."