The P6 Professional database normally handles restoring select deleted data using a safe delete setting. While using P6 Professional, the Undo command (Edit, Undo) allows users to restore certain types of data that have been deleted. Deleted data remains in the P6 Professional database until the CLEANUP_LOGICAL_DELETES procedure clears it (after 5 days, by default).
See the P6 Professional Help document for more information about using undo.