If you had saved it at all while typing it, click on the Revisions module in the post editor and you may find your post there. If you can't see the Revisions module, click the 'Screen Options' link at the top fo the Post Editor window.
Generally it's not a good idea to type a whole post without clicking on 'Save'. You should also know that the auto-save only kicks in when you've saved for the first time manually. So, best advice is to type one word or character and then save, then continue. Keep saving.
Also you can compose offline and copy and paste into your post (though be aware that if you do that in Word, you need to use a specific button on the editor to make sure the program's special formatting is removed).