HOW DO I GET MUSESCORE TO PLAY WHAT I'VE WRITTEN?

First of all, I'm very tired. I'm in advanced mode. I've tried choosing a starting note, then pressing the "space" key. I cannot get the actual playback icons to "light up" so they are available to be used. As I said, I've used MuseScore for years, and have played back what I've written many times. It's been long enough, apparently, that I've forgotten. I've gone to the "Online Handbook", and did what it said to do. Still, no "active" playback icons are available. I KNOW that there's a simple solution. Please....HELP! :)