You can view the upload progress of your videos in the "Videos" tab.
If the app isn't open and running in the foreground while your video is uploading, it's possible that the upload might pause. This happens when background processing is disabled by the device's operating system or when network connectivity is lost. The upload will however be resumed the next time the app is opened as long as data transfer is allowed ie. the app is connected to wifi or a cellular network (if enabled in your settings).
Keep in mind that incomplete uploads do expire. If your video hasn't finished uploading within a 48 hour period the app will have to start again.