You may need to uninstall the email app, yes uninstall, then restart the phone (turn it off and back on). Once back on, then go to Play Store and reinstall the app.
If this doesn't resolve it (which it should), then you are looking at a "Hard Reset", which you really don't want to do if you don't have to.
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the phone.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...
1. Make sure that the phone is actually powered off.
2. Remove your SD card, if there is one in the device.
3. Press the...