Right clicked on what? Do you mean "tag" as in colouring selected messages, or by tag do you mean some other part of the user interface? What does clicked on the "inbox again" mean?

I really can't decipher your description to understand what you've done, or what you are now seeing. Try again, or post a screenshot, please.

https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem

Please explain a bit better. Is it the inbox-folder thats disapear or the contents in the inbox? Check if your filter (unread) is still on? if you cant see the folder-pane: (press F10 or Alt if you cant see the menu-bar) on menu-bar choose: view / Layout / Folder pane

rod247 said

When I right clicked on to find the tag unread. I by accident clicked on the "inbox again" the email disappeared. Where has it gone? and how can i retrieve it?

OK I have several email address's. I click onto an email inbox. It shows the emails that have arrived. With the cursor highlite an email. Right click and a menu is shown. Open message in new tab, Move to, Mark, and so on including, Inbox again. If you click this the email disappears. Where has it gone?

somebox again refers to your last move.