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It is possible that your email lost connection with server. First we can reset your iPhone and see if it works or not. Press and hold down the Power button and Home button at the same time until it shows the Apple logo. When you see the Apple logo, release the buttons and let it boot. Check Mail.
If that didn't work, then go to Settings tap Mail, Contacts, Calendars. Tap your email account and delete it (tap Delete Account). Then add it back from Settings > Mail, Contacts > Calendars > Add Account.
Please let me know if that didn't work, so we can continue troubleshooting.
Go to Settings > General > Reset and tap Reset All Settings. This will reset some settings like WiFi, wallpaper, ringtone, bluetooth and some other settings. But this will not delete any of your files or stuffs.
After resetting all settings, go to Settings > WiFi and connect to your WiFi network. Then delete the Mail account and add it back. Does it work now ?