Normally when something like this happens, it's because the time on the computer got changed accidentally. I'm assuming that you are on Windows.

Unpredictable results can occur if adjustments are made to the clock, especially moving the clock backwards in time. Different games will respond differently under those circumstances, ranging from minor statistics anomalies to killing whatever is living in the game. Virtual Villagers has a much more complex real-time engine than Fish Tycoon, and you're much more likely to see problems with it when adjusting the system clock.

There are a couple of ways to resolve the problem: You could wait it out and see if the game recovers, perhaps starting another game in a different slot while you're waiting. Or you could just delete the malfunctioning game and start over.

The clock can also be inadvertently changed if you double-click that little Windows clock in the bottom right corner to check dates and times (a lot of people do that without realizing that it changes anything on their computer). Also, Windows has the ability to set the computer's clock using an Internet time reference, so if the clock was inaccurate the resulting time correction could affect the game.