Mr. Rewrite received an e-mail saying something had "struck a cord." His heart goes out to the poor, abused cord.
The idiom is "strike(s)/striking/struck a chord."
This post gives Mr. Rewrite an opportunity to use his new American Heritage Guide to Contemporary Usage and Style. The main meaning of chord is a harmonious combination of musical notes. Drawing from that, the idiom "strike a chord" suggests a harmonious mental resonance with something other than music.
Those paid to know better do a good job with this. You'll see a handful of mistakes by following these Google News links:
Mr. Rewrite was a tad concerned to see a large number of references on Google News to vocal chords and spinal chords. However, his new usage/style guide notes that chord can be a variation of cord drawing from Latin, which in turn draws on the Greek word chorda, meaning "string, gut." This might be more acceptable across the pond, as many of these references are from the U.K., Australia and Canada.
Medical use demands cord, however. A doctor, at least in Mr. Rewrite's country, shouldn't refer to your vocal chords or spinal chord. Mr. Rewrite suggests following your doctor's example on this.