In the first movement (Adagio sostenuto) of Beethoven’s Moonlight Sonata,1 there is a natural sign (♮) followed by a sharp (♯) on F. What does this mean?
1 Piano Sonata No. 14 in C♯ minor “Quasi una fantasia,” Op. 27, No. 2
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I don't know the exact instance that you're referencing, but this type of notation basically means two things simultaneously:
The natural sign negates a prior accidental (like a flat or a doublesharp), or just reminds us that that prior accidental (if it was not in the same measure) is no longer in play.
The subsequent sharp then tells us that, after that prior negation, we make sure to play this pitch as an F♯.
In short: just play it as an F♯!
Not all engravers follow this practice, and it's becoming less and less common as we move on through history. But in Beethoven's time, this was a relatively standard practice.
You mean here? (It really shouldn't have been MY job to find the example!)
Note the Fx (double sharp) in the preceding bar. The ♮♯, though not strictly necessary - accidentals persist only until the next barline - confirms that this ISN'T a double sharp, just the single one from the key signature.
Modern usage would probably be to retain a single cautionary sharp but omit the natural.