If you added hours to the Activehours app and they disappeared, don't worry. This means your pay cycle has refreshed or is about to refresh. Since you've already been paid for those hours, or are about to be paid for them on your normal payday, you won't be able to use them through the app.
You can still add new hours to your Activehours app, though. Just make sure the hours you add are for hours you worked more recently.