The default Mail behavior for pressing the space bar with a sent message selected is to scroll to the end of that message, after which it selects the next sent message. So the reason it disappeared is a mystery.
The first thing to try when something like that happens is ⌘ z or Undo. The Undo buffer may be gone by now so it probably will no longer work.
If your email service has a webmail interface, try logging in there and determine if you can find the missing message.
The last suggestion is to select the Sent Mailbox again, and from the Time Machine interface in the Mac's menu bar, select "Enter Time Machine". Give the interface time to populate (a lot of time, depending on the number of email messages it may have saved), and determine if you can restore it. If the messages are not stored locally, "On My Mac" then TM will attempt to download them again from your email service providers — assuming they remain on their servers.
Restore items backed up with Time Machine on Mac - Apple Support