Writing a score but notes won't sound out loud

So I'm writing a song for multiple parts and it is fairly easy when I can hear the notes out loud so I can play them all together to actually hear what they sound like. But this second time I came in to start editing again and I can still add notes and rests, but I can no longer hear the notes when I click them and also I can't seem to click the play button to hear what I have written so far. My sound is on also, so that works.