Parent drop-down doesn’t appear in admin

I had created parent page – p1 and under it two sup-page c1 and c2, and it was working but after some days, I came to my admin panel I saw all pages p1, c1 and c2 are not in hierarchy means there is no parent – child relation maintained. again I tried to make c1 and c2 to put under p1 but I wonder there were no parent drop-down menu available in admin edit-page only parent label was appearing without drop-down. Can anybody help me?