Emacs: Open Recently Opened File
Alt+x recentf-mode
to turn it on. (it toggles on/off)
Then, Alt+x recentf-open-files
to list and open recently opened file.
To open the file, just press a digit key. Or, move cursor to the line and press Enter.

Turn on Recentf for Future Sessions
Put this in your Emacs Init File:
;; keep a list of recently opened files (require 'recentf) (recentf-mode 1)