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Dear Leva,
Welcome to the forum and letting us know the situation in your environment.
From the given information, it seems that the list data isn't showed properly after the list UI is changed in your environment.
I understand your current feeling and your fear of the data loss after a long time of the heavy work. As the issue occurs after the list UI change, I think that the data may not be fully deleted and some UI issue may happen in your tenant which causes that the data disappears.
As the list UI is deployed to all tenants by the product developers, I'm sorry that our category may have limited resources on helping you fix the issue.
However, please don't worry, as a tenant administrator, he can phone or open a support ticket to the related team for fixing the issue from their side directly.
If you aren't the administrator, I suggest you contact the administrator in your company and let him follow the steps in Get support - Microsoft 365 admin | Microsoft Learn to phone or open a support ticket to the related team to report the issue to them as soon as possible.
About admin roles in the Microsoft 365 admin center - Microsoft 365 admin | Microsoft Learn
How do I find my Microsoft 365 admin? - Microsoft Support
The administrator should be able to open the support ticket from Help & support in the Microsoft 365 admin center.
Then he can select the Phone support or the Email support as follows.
I hope that the issue can be soon resolved in your environment.
Your understanding is greatly appreciated!
Sincerely
Cliff | Microsoft Community Moderator