In my case it was Bach's Passacaglia in C minor played on a carillon. It is so beautiful I had to listen again, went to sleep with it going round in my head, and woke early to listen again again. Then it was stuck in my head all morning.
I don't know what makes this so good and so moving. Perhaps it's the idea of this music, which with its repeating bass theme to me just speaks of continuity, gently descending from above, anonymously from a church tower. Perhaps it's the minor thirdy-tone of the bells themselves that complements the melody. Perhaps it's also the stunning technique, which I almost failed to notice (true genius is making complicated things so simple no one notices). Perhaps also it's the surprise value. I don't know.