Hello, Leo1956.
Thank you for visiting Apple Support Communities.
I would start with closing all open applications as outlined in the first article and power cycle the device. If the issue persists, the easiest way to troubleshoot this issue would be to delete the account and add it back to the iOS device. Below, I have included two additional email troubleshooting articles. The second article covers basic email troubleshooting. The third article will assist you if the email account you are experiencing issues with is a Gmail account.
iOS: Force an app to close
http://support.apple.com/kb/ht5137
iOS: Troubleshooting Mail
http://support.apple.com/kb/TS3899
iOS: Gmail account will not connect to Gmail server
http://support.apple.com/kb/TS3058
Cheers,
Jason H.