How to fix a dead note?
 

How to fix a dead note on a guitar

Dead note, or dead string? If the entire string is dead (I'm assuming you've changed strings) it's likely a worn saddle or nut. If it's just one note, it could be a worn saddle, nut, or worn fret. Or, your action might be set too low. In any case, take the guitar to a reputable guitar store. They'll be able to tell you exactly what's wrong and offer advice on repair.

What note is it and what location on the fretboard?

I "assume" by your post that it may be a fretted note. If so, then you likely have a loose or improperly seated fret. Remove the strings, and tap a few frets with the end of a plastic handle screw driver. Each fret should sound the same. A loose fret will have a dead sound compared to a seated fret. If you find a loose fret it can be tapped back in with a hard plastic faced hammer. Support the neck over a bag of shot gun "Shot" and tap the fret in lightly. If the fret is over the top then you have to support the frert board extention by holding a heavy piece of steel tightly under the FB extension BEFORE you attempt to hammer in the fret.

If the fret pops back up after hammering it then it will have to be glued down with super glue.

Before you assume that it is a loose fret try tuning the string down 1/2 tone and see if the dead spot moves up 1 fret. If it does, you do not have a loose fret. What you have in that case is a common occurence in many instruments where the resonant frequency of the body of the instrument is the same as the note you are playing and they tend to cancel each other. That is why the note sounds dead. It is a natural acoustic phenomenon that even some very good instruments have and in fact may be more noticeable on high quality instruments
You cannot change the laws of physics.

If the dead note does not move when you tune down then I would look at the loose fret possibility.

Aaron

Aaron has given you the most likely reason. The common name for the phenomenon is "Wolf Tone". Every acoustic stringed instrument on the planet has this effect, though in some individual instruments it is more or less pronounced than in others. Below that note on your fretboard, your guitar body and the string are vibrating in phase. Above that pitch, they are moving in counter-phase. The Wolf Tone is point at which the acoustic turbulence is transitioning from in phase to counter-phase. It is the threshold point. You can design an acoustic guitar that will minimize the Wolf Tone point by adding heavier bracing to keep the body from vibrating so much so you don't get such a pronounced transition but when you do that you start to lose the sensitivity and complexity of tone since your instrument is less lively. A guitar's designer has to make a tradeoff in their design, and the player has to make a decision if they want a dull and dead sounding overbraced acoustic with the Wolf Tone being almost silenced, or a lively and complex sounding acoustic with a noticeable Wolf Tone that they try to avoid hitting in their playing.

My daughter plays cello - these wolf tones truly are on a lot of string instruments. She uses clip-on weights below the bridge on the offending string, obviously something you can't do on a guitar (unless it's an archtop).
Best wishes.

Sometimes it's the geometry and flexibility of the guitar, the neck, and strings. I notice on some guitars the 7th fret on the A string is more muted. That's an E note. Whereas the same E note on the D string sounds fine.

You can find this to some degree on a lot of different guitars. Some sizes, shapes and makes seem to be more pronounced. Different body sizes and the dull notes will be in different places. I have traded off a couple guitars which were so bad in the middle of where I play a lot, that I just couldn't live with it.
Like most have said, it's there somewhere and to some degree in most instruments.

I've had two gorgeous PRS Electric guitars. One a Custom 22 and the other a ten top Mc Carty. Both had dead spots in the middle of the neck somewhere around the 8 or 9th fret. Both were very pronounced and drove me nuts. Other than that they were both great sounding and playing guitars. My Martin HD28 is the first acoustic I've ever had that didn't have any dead spots that could be heard while playing. Maybe the syndrome is more noticable on an electric than acoustic.

This thread is turning into a perfect example of what the Internet is not good for...diagnosing problems with guitars. What you see here are two very different lines of thought, both coming from reasonably savvy people.

You should take this guitar to a luthier who can very quickly determine if it's a fret problem. If it's a wolf tone issue (and really in the violin world the wolf is a loud note, not a dead one...) or more accurately a resonance cancellation issue, then you may be stuck with it.

There are a few possible experimental remedies...1) adding or subtracting mass to the peghead in the form of heavier or lighter tuners or using a Groove Tubes "Fat Finger", 2) adding in some additional bracing in the top in just the right place, or 3) stiffening the neck with carbon fiber...which involves taking off the fingerboard.

Back in my repair days I did all of the above to guitars, and #2 and #3 both worked pretty well, but were time consuming and expensive. #1 did more about overall sustain.

I must be deaf or lucky. I've played a lot of guitars. Many of them pretty high end and lightly built. I play all over the neck. Not all that well, mind you, but I'm not just playing cowboy chords. I have, of course, run into "dead" notes on a few guitars, but nothing so severe that it couldn't be compensated for pretty effectively and pretty easily. I'm sure there are guitars out there, even very good ones, that have notes that just go stone dead, and can't be worked around, but I'm not running into them. This is an evasive mystery for me, right up there with the magical guitar that "blows away everything else I've ever played" (typically someone's most recent purchase :)). Sometimes I feel like I should work in a guitar shop in my retirement so I can experience these great mysteries and rare finds!

;)

Tim