If you just deleted the message and have not done anything else, try giving the iPhone a firm shake. In many cases, you should see an Undo Trash alert on screen giving you the option to reverse your last action — accidentally deleting the message.
For situations when shaking the iPhone is not going to do anything but illustrate your frustration, you can manually fish the message out of the trash and restore it to your inbox. Start by opening the Trash folder for the affected mail account, then locate the message and open it. At the bottom of the screen, tap the folder icon. A list of all the folders for that mailbox appears. Tap the Inbox icon to transfer the message from the Trash back to the main Inbox.
If you have multiple mail accounts on your phone and are not sure which one received the message you want back, you can add a universal trash folder to your list of mailboxes. On the main Mailboxes screen, tap the Edit button in the upper-right corner. Scroll down the list of current and available mailboxes and select the All Trash option. Tap the Done button to return to the main Mailboxes screen, where the new All Trash mailbox is waiting for you.
It can be very easy to hit the wrong button and delete a message when notification alerts are popping up on your screen. If you want to change the style of the alerts you get for new mail, open the Settings icon on the home screen, select Notifications and scroll down the list to the Mail icon. Tap Mail, and on the next screen, select a mail account to get to the iOS notifications for new messages.