Solutions:
1. EKGraph battery is not charged.
2. Press and Hold FIRMLY on the power button for 3 seconds.
3. Automatically the screen will turn on and you can start using it.
Like in this GIF
https://giphy.com/gifs/ja93ARRiFdNsNG5twc?utm_source=iframe&utm_medium=embed&utm_campaign=Embeds&utm_term=https%3A%2F%2Fsonohealth.freshdesk.com%2F
Stay Safe and Healthy!