A lot of pianos, including concert and rehearsal pianos are played vigourously causing a lot of stress on the strings.
Strings believe it or not, have a lifespan. What is the lifespan of that string is over, it breaks and needs to be replaced.
What’s the piano string is replaced, the piano is then tune and his back to its brilliant original sound and tone.
If you have broken strings on your piano, don’t hesitate to call and have it repaired and the piano tuned.