This happened all of a sudden. I could not start the 3g on my phone.
The Data Network mode is always deactivated now.
Although, when I go to Settings > Wireless and network settings > Mobile network settings, the Use packet data option is checked.
Unchecking this option and checking again doesn't work.
Installing the internet settings again from the service provider also didn't help.
Restarting the phone didn't help.
Restoring factory settings didn't help.
Any idea what's going wrong?