Hello Krishnan and Welcome to the Forum.
The first thing I would do is go into the attic and pull the insulation batts out in that area and lay them aside to dry out. Then, while you're in the attic, you should look straight above the leak area and see if you can see the source of the leak. Another thing to do is look up the slope of the roof from the underside and see if there is a chimney, vent pipe or other roof penetration; or a valley in the roof. All these items are notorious for causing leaks. Then go up on the roof and check the vent flashings and valleys to see if you can remedy the cause. When I put a flashing on a plumbing vent or a gas vent pipe I never put any nails in the lower edge of it, if it does have nails in the lower edge they will need to be coated with plastic roofing cement or silicone caulking, whichever gives the cleanest look from the ground.
If you have so-called "California" valleys; may heaven help you. They are a royal pain. In a CA valley the roofer puts his shingles on the right side of the valley allowing them to run wild through the valley about a foot; then they roof the left hand side allowing some overlap in the valley agiain, then they chalk a line up the valley and trim the left side shingles to the line. Some of them will put a gallon or so of plastic roof cement under the left shingles to seal the valley.
If you look on a bundle of shingles the complete instructions are right there; and the roofing manufacturer will not cover any variations from their instructions. The bundles will also have a "Limited Warranty" on them; believe me that word limited has much more weight than the word warranty.
As for the ceiling, you may have to cut the damaged sheet-rock out and replace it, then try to match the ceiling up the best you can.
Glenn
