If your Leaf is not syncing and your alarms or inactivity alerts are not vibrating, it might be possible that your battery is empty. If you haven’t changed your battery in the last few months and the battery indicator is showing that your battery is around half full, it might be a good idea to change it. You can learn how to change the battery here.
Once the battery is replaced, the Leaf app might still show a low battery warning, but this shouldn't worry you. All you need to do is sync your Leaf a few times, and the warning should go away.