That's pretty upsetting to come back to that kind of disaster. The most common reason for it, though, is that your farmer(s) and/or healer(s) got sick, and there was no food being gathered. It doesn't take a long time (on fast speed, as Lurkily said) for really bad things to happen.

It's probably too late now, but the game log could tell you why they died, if it hasn't been overwritten. This answer in our Support FAQ has details about using it.