Sounds like something might’ve changed in your Steam settings. This happened to me once. Try going into Steam’s settings, then head over to the Controller section. From there, click on “Guide Button Chord Configuration” and make sure “Guide Button Focuses Steam” is checked.
If that doesn’t fix it, you might want to check if Steam is set to run in Big Picture mode when you start it up. Sometimes updates or changes can mess with these settings. If all else fails, try restarting Steam or your PC—it’s a cliché, but it works more often than you’d think.