When you open a map, the Mapping Editor automatically deletes mapping rules from the map in two cases:
Schemas can change due to changes in the connection activities or message definitions they are generated from. Schemas that you add to the project can also be edited directly.
Review the map and recreate or alter mapping rules to handle the schema changes properly.
In this case, the Mapping Editor deletes all mapping rules, and the map cannot be recreated. Close the map and delete it from the project.