Your Mendix Free App goes into Sleep Mode after an hour or so of inactivity and automatically resumes when a user accesses it. All your data is retained while the app is in Sleep Mode.
Perhaps you have accessed your app within that hour?
To make sure what kind of license your app is using: go to sprintr.mendix.com. You see something like this for a free app:
When your app has become a licensed app, like this:
Now, for finding out why your app has not gone to sleep: do you have any published rest-services that someone calls?