This sort of thing often happens to people who ride but don't do much with their bikes otherwise .... no one will tell you what to check periodically, like the condition of cables, or the tension of the spokes. When things get bad and are left unrepaired, they get worse and worse until ... as in this case .... something minor actually breaks.
As people have said, in all likelihood you just need the spoke replaced and the rim trued and tensioned. I think my local shop would charge about $75 .... no clue where you are or what the cost might be, but even if you buy a new wheel, you would want to take it in a get it tensioned after a week or three.
If it is a front spoke you can easily replace it yourself, but you need to know the spoke length. if it is a rear spoke you will likely need to pull the cassette ..... and in both cases it would probably be wise to have a shop true and tension both wheels.
Once the wheels are again straight and round, you can keep track of them just by tapping each spoke---they all should make about the same sound. if a spoke is loose it would ring with a lower pitch, so you could tighten it yourself or at least know it needed attention.
Once the wheels are right, they should last for a long time .... years, if they are built well.