Touched a faint = vague, unclear, etc. Long-lost chord = In this I would think it's a memory or an emotion. Some sort of feeling or memory that was unclear.

"To strike a chord" literally means to play a harmonious group of notes on a musical instrument... a C major chord, a G-seventh chord, etc. Figuratively it means "to evoke an emotional response." "What he said in that speech really struck a chord with me." In this case, it is a "faint, long-lost chord." She evokes some kind of nostalgic memory. It may also be a reference to a once-famous song by Sir Arthur Sullivan, "The Lost Chord," about a person seated at an organ who accidentally strikes a wonderful chord, and never can find that chord again.