There are now more than a few hundred email services out there from gmail, yahoo and the other hundreds. All it would take is for the email service to change their settings and your device to not be setup quite right.

The sending and receiving settings are different so it is possible to send but not receive. But here's the rub, setup is all over the map with no two email services being the same. So there is no exact advice except to check settings and call your email provider.
Bob