The problem didn't go away after closing and reopening the Mail App either, and when I had network connectivity again, the email stayed blank.
To fix this, the mailbox needs to be rebuilt. This is done from the Mailbox menu by clicking Rebuild.
The Mail App will re-download all the emails for the mailbox, it can take a bit of time, so it's important to let it finish doing what it's doing. There is an indicator at the bottom of the window to show its progress.
After the progress bar goes away, showing that Mail has finished the rebuild of the mailbox, some emails will still come up blank, but in a different way (the background will be grey). This is ok, just leave it for a while and the email will appear once again.