Dear Kotryna Lengertaite,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
Based on your description, I understand that you have a query "Why my hours of work suddenly disappeared?."
I'm sorry to hear that you're experiencing issues with your Microsoft Teams assignment. Have you checked if the assignment is still available in your Teams channel? You can do this by going to the channel where the assignment was shared and checking if it's still there.
If the assignment is still there, try opening it on a different device or browser to see if the issue persists.
If the assignment is not there, please check if it was accidentally deleted. You can do this by going to the "Deleted" folder in your Teams channel and checking if it's there.
If the assignment is not in the "Deleted" folder, please contact Microsoft support for further assistance, Get support - Microsoft 365 admin | Microsoft Learn. They may be able to recover the assignment from a backup or provide additional troubleshooting steps.
I hope this helps! Let me know if you have any further questions or concerns.