I've never ran into this problem before....

I was playing single player earlier today and during a 1 hour session, a message prompts up saying "Problem connecting to network" and then It throws me to the main menu. So I alt tab and look at my network connection and it has been disconnected. What I don't understand is isn't this a single player game so it shouldn't make a difference whether im connected online or not, and I've never encountered this problem during the many hours playing this game. I went to re load my game but I found out that an hour of gameplay was missing :(. I was wondering have any of you guys encountered this problem?

Thanks in advance.