@Vijayakumar Somasundaram Thank you for reaching out to Microsoft Q&A, apologize for any inconvenience caused on this.
Based on the shared information, I have looked at the backend logs and we see a delete operation on the Notification Hub (WiAtNS) on 3rd January and it got successful as well.
If you want to know more why you have received the above error while sending notification to your windows devices, I suggest you raise a support ticket to check further on this.
Also, try creating a new notification hub and check and if you still facing the issue.
Feel free to reach back to me if you have any further questions.