Hello,
I created my first recording in Audacity, and I have two tracks: one is voice and the other is music. This evening when I opened my file, I could only see the music track. I opened all previous versions of the file, and on all of them I can only see the music track.
Can anyone please advise how I can view the voice track again?
Thank you.
I do not believe you are going to find any “hidden” tracks. 
So the question is why were they there and what happened to them? If you had two tracks on the audacity screen and you "SAVE"d the project, they should both be there when you open it again. That is a mystery.
So I am thinking you "EXPORT"ed the file, perhaps as a WAV or as an MP3. When you export two mono tracks, Audacity combines them into one mono track for the export. When you export two stereo tracks, Audacity combines them into one stereo track.
There are simple ways to combine two mono tracks into distinct a distinct left and right channels, see Splitting and Joining Stereo Tracks - Audacity Manual
but I am not sure this is what you want to do. I hope this helps. 
Thank you very much for your reply. You are correct. What happened is that the two tracks are now combined. And yes, I did export the file. Is there any way I can recover the two distinct tracks?
The short answer is no.
The long answer is probably not.
Audacity has an effect “Vocal Reduction and Isolation” that might help you. Your chances are better if the original music track was stereo and the vocal was in mono.
If you have the original recording of the music (or vocal) saved as a .WAV and your exported combined file was also a .WAV file, you could try "Invert"ing one track and then recombining.
Good luck.
Thank you again, Jademan.
But I’m still a little confused. I just created a sample recording, which I named, Sample.aup. It had one track of voice and one track of music. Then I exported it to an .mp3. Then I opened the .aup file again, and I still have two tracks.
So why did it in one case combine my two tracks, and in another case it did not?
Yes, that is correct.
Your SAVEd .aup file is going to retain all of the tracks unless you delete them.
The EXPORTed .mp3 or .wav file is going to have all of the tracks combined.
Changing the subject, you could change the balance on the music track all the way to the left, and the balance on the vocal track all the way to the right, then if you EXPORT, you should have a single track, dual channel stereo file with the music on the left and the vocal on the right. 
Thank you. For some reason in my original recording, in the .aup file, the tracks got combined. But in my test file that didn’t happen. I decided I needed to rerecord my original one anyway, so it’s not too serious.
Thank you again for you help, and have a great day!