I don't remember changing any settings that would have caused it, but day before yesterday I noticed the 3G icon on the notification bar was gone and nothing that uses the internet is working. I can turn on 4G and that works fine but when I turn it off there's no data. In Settings / Wireless & Networks, Mobile Network is checked, but it's grayed out and won't let me change it. Restarting doesn't help. I tried changing the setting for data roaming but that doesn't help. Any ideas?