If the wearable is showing as offline, check that it has a battery by tapping the screen to light up, if it does not, put the wearable to charge and it should be online again. The wearable will take around 1 hour to get a full charge.
If after charging you still can’t see the current heart rate, movement levels or time, first try tapping on the screen, which should wake it up.
If that does not work, you can long-press the bottom of the screen for around 4 to 5 seconds, to turn it on. This should cause the wearable to vibrate, and then light up.