If there are nail heads that are not currently covered by anything if multiple rows removed then create new holes in the roof to hold the shingle by moving the nails over.
How to repair a 3 tab shingle roof.
Removing 3 tab shingles using a flat bar to do a repair.
Once the nail pops up about 1 4 in remove the pry bar press the shingle down and pull out the nail.
Then push the pry bar under the shingle directly above the damaged one and remove the second row of nails the same way.
Removing 3 tab shingles using a flat bar to do a repair.
Repeat this procedure for the remaining three nails.
Press tabs down into the cement firmly and secure the shingle into place.
However if that is not a possibility then your second best option is to find rolled roofing once called 90 lb.
Lift each tab slightly on the shingles as you install them and apply a 1 2 5 cm diameter spot of shingle cement under each tab.
The best way to repair a t lock shingled roof is to have it replaced typically this will be covered by insurance.
Reseal under edges of all three shingles.
Make sure it looks right and lines up properly.