Hi ff100, welcome to the forums!
You've been hit by the random freeze bug; this is the best-known bug in the game, and unfortunately there's nothing you can do to avoid it other than to save regularly and reload the game when it happens.
Mike (The developer) has fixed this bug in the next update - which should actually have already been released by now. There's been some sort of delay, but I haven't been able to contact him to find out what the cause is. We'll post news of the update on the forums as soon as the update is available. The game will also notify you of the update itself because it contains a built-in auto-updater.