Two things to try...
1) Force-close your Mail App (swipe-up from the bottom of the screen to reveal all “open” processes, then swipe the Mail App of the top of the screen) - then force-restart your iPad (press and hold both the Sleep/Wake and Home buttons until the Apple logo appears on screen - then release both buttons). This often cures many unexpected ailments.
2) Assuming that your linked mail accounts use IMAP to connect with your email provider (therefore all email remains on email server until intentionally deleted), you can safely remove the account(s) from your iPad - restart the iPad - then re-add the email accounts to your device. This process will clear any local copies of the email from your device, then download them again (with all header information intact) from your email provider.
The first option is very quick and may resolve the problem. The second option takes some time, but if the first option is unsuccessful, the second is very likely to provide effective resolution.
I hope you find this helpful in resolving your problem.