If you leave your Roomba to do its thing unattended—which is the whole point, after all—every once in a while it can get stuck in a corner or under furniture. If you can’t figure out where it went, and you have a Wi-Fi connected model, you can ring it from the app to help you track it down.
To find your Roomba, open up the iRobot HOME app on your phone and tap the three dot menu button in the bottom right corner of the screen.
On the next screen, tap the Locate button. This will cause your Roomba to start playing a little song (assuming it has enough battery life left to do so).
Once your Roomba is ringing, walk around your home until you find it and either set it back on its way or carry it back to the charger.