It's likely too late now, since the game log has probably already been rewritten, but this could help you next time a villager dies.

You can check the game log to determine exactly why your villagers died, as well as to learn many other interesting bits of information about what your villagers were doing while you were away. The game log is recorded in a file called ldwLog.txt and is located in the Virtual Villagers folder in Program Files (for Windows) or in the game folder (for Mac OS). If you have trouble locating the game log, search for ldwLog.txt using your operating system's search function. The game log can be opened in any text application, such as Notepad in Windows or TextEdit in Mac OS.

A villager dies whenever their remaining health reaches zero. Villagers can also die of old age, even when their health is not depleted.

The game log can be very helpful in such situations, if you check it before it's overwritten. The game log is overwritten each time you launch the game, so it has to be viewed immediately after you quit the game. If you wish to preserve it for later viewing, you should rename it or copy the file to another location.