If you are using Mountain Lion, mende1's suggestion works. But I think you're using Lion because of the "save a version" command. If you're using Lion you need to go to File > Duplicate & then File > Save.
If you have previously saved the document, & I hope you have since it's "a large document," autosave in Lion or Mountain Lion will save it for you periodically.
Thank you both. I am using Version 10.7.5 and performed a "file save" before dinner then, after dinner I found the "save" and "save as" options no-longer visible. Very strange, although if I click on the red "close" button at top-left any changes are saved.