If You Have An Apple Device:
If the Owlet App is unresponsive, but your iPhone, iPad, or iPod touch still reacts to regular commands outside of the frozen app, you can close the frozen app as follows:
The following instructions are from https://support.apple.com/en-us/HT201330
View your open apps
Double-click the Home button to see your most recently used apps.
Find the app to close
Swipe right or left to find the app that you want to close.
Swipe up to close
Swipe up on the app's preview to close the app.
From your Home Screen, press the rectangular button that will show the list of currently open apps.
Find the Owlet App and swipe to the side on it to close it completely.
Once You have Force Closed the app, wait a few seconds and then open it back up to re-launch.