If the PowerCenter repository is a versioned repository, it is possible to rollback changes to previous (checked-in) versions.
User gets an object (such as Mapping, Workflow or Transformation) deleted then it is no longer available through the GUI
but all versions are still in the repository.
To recover the last version if you deleted an object you can first check if the object is still checked out:
If the deleted object was already checked-in, you can recover the object by using the Repository Managers Query tool: