You should first check to make sure that your game is not paused; however, this problem is most frequently caused when people try to make the game progress more quickly by adjusting their computer's system clock. While that sort of "time travel" is a popular way to speed up game play, real-time games rely on the accuracy of the computer's system clock to operate properly. 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.
While many people do adjust their system clocks to speed up play, doing so affects far more than just the games. Every application in your computer is affected, including the file system and operating system processes. For all of those reasons, we do not recommend changing your computer's system clock to speed up game play.
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 use that little clock in the corner to check dates and times (a lot of people do that without realizing that it changes anything on their computer). Also, every recent operating system 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.