1. Maybe you've used up all your gigabytes. It happens.
2. Maybe you're a foreigner in the land without a local SIM card. Nobody data roams, except business travellers who don't give a shit about their employer.
3. Maybe your carrier signal is unavailable, or they're experiencing issues on that day; it happens.
4. Maybe you're in a city, like London, which doesn't have carrier signals in the underground/metro/tube.
