Reading nano man page, and some search, I found:
In some cases nano will try to dump the buffer into an emergency file. This will happen mainly if nano receives a SIGHUP or SIGTERM or runs out of memory. It will write the buffer into a file named nano.save if the buffer didn't have a name already, or will add a ".save" suffix to the current filename. If an emergency file with that name already exists in the current directory, it will add ".save" plus a number (e.g. ".save.1") to the current filename in order to make it unique. In multibuffer mode, nano will write all the open buffers to their respective emergency files.
So you should maybe already have such a file waiting for you, somewhere on your system.
find /likely/path -mtime -1 -print | egrep -i '\.save$|\.save\.[1-90]*$'
(/likely/path being first the place where you launched nano from, then other such "possible" places, then in last resort:
/ (of course, launch that last find command as root or expect a lot of error output, which you could redirect away using your shell's STDERR redirection)
-mtime -1 says "up to 1 day old", you may want to change the value to -2, or -3, depending on when you edited the file and when you read this.
In the event nano did not yet write such a file, you could try to send it a SIGHUP signal to force it to do so (see: http://en.wikipedia.org/wiki/Unix_signal#POSIX_signals )
And then, run the find again to look for that file...
And in last, last resort, you could play with grepping through /proc/kmem for parts of the text you are looking for, but this will necessitate some precautions to sanitize what it shows you, and could be not trivial. or dd it first into a (as big as your memory) file.