Click the cogwheel (top right).
Select 'See all settings'
Across the top of the page, you'll see lots of tabs. (The 'General' one will be pre-selected). Click on 'Labels'.
Next to 'All Mail', click on 'Show'.
Now move your mouse across to the left of the page, so that the list starting 'Inbox', 'Starred', etc becomes visible. Scroll down to 'All Mail' and click on it. You should then be able to find the missing email.
(The above is an example of why I hate Gmail. It makes everything so ruddy difficult!)