I've got a Yamaha classical guitar. Not sure of the model but its under a year old and has CG111S on the sticker.
Anyway, I woke up one morning and saw that the A string had all the metal wire unravel itself from the rest of the string.
The rest of the strings are what came with the guitar. Do I need to replace all the strings or just the broken one?
What gauge string should I go for? Is there a standard for classicals?

BTW, it has nylon strings.

Since they're nearly a year old, I'd say it's high time that you replaced them all. Typically, nylons last for considerably longer than a set of steels will. However, you've still got the wrap wire to contend with. And that will tarnish and oxidize just as fast as any other string. Pop for a new set and away you go. Savarez would be my choice. Medium to high tension.