If you have clicked 'no' by mistake to save changes to and Excel document can you still retrieve this file?
That depends on the specific situation. If the document has been
previously saved, you will have the file up to the point it was
last saved, but without any of the new changes in it. If it is
auto-saving you may have some more. If it is a brand new document
that was never saved before, then you will have lost everything. If
there was an error in Excel as you were working on it, you might
get a recovered document, but if you just said you did not want to
save it, as you've indicated in your question, then that won't
happen.
As a general rule, if you are working on a big document, save it
every so often, even before it is finished, so that if you do
something like you did by hitting "No" you will have at least got
some work back. Make sure the Auto save is on, by going into the
Tools menu and then Options and then in the Save tab, you can turn
on autosaving. That will help recover a document if there is an
error in Excel, so you won't have lost everything.