It's nowhere to be found? Not in the place where you saved it?
After initially saving it (Save as), did you use the Save operation?
You could use a Find utility that also searches the hidden folders, such as EasyFind.
There are some instances where people have used Save and then their edits didn't get saved. I don't remember one where the file got lost completely.
If you still cannot find your file, then it might be gone for good.
What you should do:
- Check your harddisk for failure
- turn on Time machine and use it
- Make a backup strategy and regularly save your files.
- save early, save often, save different versions.
In Illustrator it might be a good idea to turn off "Save in background"