Evernote files are kept in a database file ending .exb.
The first part of the file is your Evernote username - i.e. graham1234567.exb
Example: On my computer (Windows 7) my Evernote file is in the folder C:\Users\graham\AppData\Local\Evernote\Evernote\Databases
To find your Evernote database change graham to your whatever your username login is.
Or simply search your computer for .exb files.
NB: On another note I am surprised and concerned that it is just one file, as this will vulnerable to corruption. Are there any nightmare stories of people losing their whole endnote files to corruption?