Strings on a piano break sometimes just as they do on other string instruments. One repair that can be done is to splice the string back together with a tuners knot. This is often a better repair than replacing the string because it can be done immediately. Bass strings have to be made to order for each piano so that takes time. Also a spliced string settles back down to tuning stability quicker than a new string does.

I know it seems strange to picture tying a knot in piano wire but this can be done and creates a good repair.