Try this: (pay close attention to the pics for the location of the component)
As unlikely as it may seem - this actually worked for me after losing signal following a screen replacement - and I had searched and tried EVERYTHING else first (at least a hundred different pages of tutorials, guides, videos - you name it). If I hadn't done so much reading though, I wouldn't have come across the solution, but it was a needle in a haystack. Frankly I am totally astounded that the fella who posted it was ever able to locate the problem; the piece is REALLY tiny and I never even knew it was ever there, much less that it was missing until I had tried everything else under the sun.
I finally stumbled onto his post in a comment from another fixit page with hundreds of people with the same problem and no fix. I had been so careful, I really didn't think that could be the problem, but when I got out a magnifying glass and studied the mobo, sure enough that little inductor was missing! A blob of solder connecting the two contacts it rested on is generally enough to regain the signal (some say even stronger than previously before it was lost). A drop could possibly dislodge it, I would think, because it seems to happen to so many who have opened the phone to change the battery or housing - possibly Apple designed it that way to keep a monopoly on the repair service.