Again from MS:
- After a clean installation of Windows, your Microsoft Teams chats may appear missing because a clean install removes all previous programs and associated data, including the local cache for Teams. However, your chat history is likely still stored in the cloud and should reappear once the application resyncs.
How to Get Your Chats Back
- Check your internet connection: Ensure you have a stable internet connection for Teams to sync your data from the cloud.
- Log out and restart Teams: Sign out of Teams completely (right-click the taskbar icon and select Quit), and then restart your computer.
- Use the web version of Teams: Check if your chats are visible in the web version of Teams to determine if the issue is limited to the desktop app.
If your chats are not still in the web version of Teams, you might consult MS support