Assuming that this is on an iOS device as you don't give any information, try turning the account off and on : Settings > Accounts & Password (Settings > Mail > Accounts on iOS 10 ; Settings > Mail, Contacts, Calendars on iOS 9 and below) , then tap the account on the right, slide Mail to 'off', exit settings and go back into the Mail app, and then go back to Settings and slide Mail back to 'on'
If that doesn't fix it try closing the Mail app via the device's multitasking bar : double-click the home button to open the taskbar and then swipe or drag the Mail app's 'screen' up and off the top of the screen to close it, and click the home button to close the taskbar.
And do a soft-reset/reboot : press and hold both the sleep and home buttons for about 10 to 15 seconds (ignore the ‘slide to power off’), after which the Apple logo should appear and the device should reboot.