Undo Close Tab in Vim

:tabnew#

Reopens recently closed file in new tab

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Edit: Please use greyfade's answer. I don't like my answer, but I'm keeping it here for references and useful comment info.

Your file is probably still open in a buffer:

:ls " get the buffer number
:tabnew +Nbuf " where N is the buffer number

To reopen buffer 18, for example:

:tabnew +18buf