Sounds like you have two problems:
1. You can't find the project you worked on
So you mixed it down then closed S1? And when you re-opened it, it did not show that song in the list of recent projects?
There is a File/Recent projects option you can try.
You can also create a new project and this time take a look at the folder it is defaulting to
2. You can't find the mixdown of the song you worked on.
When I Song/Export Mixdown (I'm not sure about your comment "send it to master or mixdown"), it opens the folder into which the file was created.
As suggested above: create another project with any audio. Song/Export Mixdown (pay attention to the location and options regarding what to select and the format). This is likely the same folder your original mix was sent to.
Hope this helps,