Hello Muhammad,
The Milestones are stored in the table CPMS_INIT_TASKS but the initiatives and milestones shouldn't disappear because a start or end date was entered unless there is a filter set up to exclude them. Can you check the Show Filters of the Initiatives and ensure that there is no filter on Schedule, Budget, or Target End Date? As well if you select the option to Include Expired do you see the initiatices and milestones dispalyed?
Regards,
Graham