Web Part Error: A Web Part or Web Form Control on this Page cannot be displayed or imported. The type could not be found or it is not registered as safe.

Error Details:
[UnsafeControlException: A Web Part or Web Form Control on this Page cannot be displayed or imported. The type could not be found or it is not registered as safe.]
  at Microsoft.SharePoint.ApplicationRuntime.SafeControls.GetTypeFromGuid(Guid guid)
  at Microsoft.SharePoint.WebPartPages.SPWebPartManager.CreateWebPartsFromRowSetData(Boolean onlyInitializeClosedWebParts)