Sometime in February we noticed part of our roof appeared to be sagging (maybe a 2 1/2'x4' section). Obviously that's not good and suggests that water is getting under the shingles and rotting the wood underneath. I got up on the roof over the weekend and confirmed that it is definitely not firm as a good roof should be.
So. The question that I have is to what degree should I expect to be into this? I've asked for a couple of opinions and received advice that ranges from "expect to replace a significant portion of the roof, maybe the entire roof" to "find where the water is getting in under the shingles and fix that spot. The roof is fine otherwise, just let it dry out and the integrity of the wood will return."

-At this point I have no idea what to do and what to anticipate. I was hoping some CBer's with some roofing expertise might chime in, TIA.