When I came back from my last travel my 3G connection stopped working and seems that a lot of people already had the same problem. It was after I reactivated my plan when I came back to the US. Apparently they forgot to activate my data plan I’m paying for.
How to solve
I tried different things but finally I tried to simply activate the phone again using the Activate application (probably the first application on the list). After a few tries It worked and I got my 3G connection back.
Improve your battery life
One interesting thing is that during my days without 3G my battery life was quite better. So when you want to save battery, in travels for example, you can manually activate/deactivate your data plan on the Setting -> Wireless & networks settings -> Mobile Networks -> Data Enabled option.