Ok, this has probably been asked a thousand times...

I put the dog out this morningm and found a roof shingle (asphalt) on
the ground.... I looked at the roof with binoculars, and found the gap,
it was sheared off flush with the next row above it; I could see the
seam and roofing nails that had been covered.
So....
will the roof leak? do I have to fix it?

the roof is 16 years old, and the shingles are "Bird Windseal 80"

I suspect I am getting near a re-roof job, which I dread, as this roof
is massive...

Thoughts?