This has happened to me too....twice!! it is really frustrating!
I am using mine on my iphone 3G and it has been at totally random points in the game (I did things completely differently the second time I started it) My husband also has Fish Tycoon on his iphone 3G yet has had no problems????

I haven't fiddled with the time on it or anything.
I really hope that a patch is availiable soon as I don't want to restart the game for a 3rd time if it is going to happen again (very disheartening!)

Kes