TL;DR: New Microsoft Teams shows a blank window or loads forever: how to fix it

The fix only. No explanation, no diagnosis. Read the full article if any of this does not match what you are seeing.

1. Sign in to Teams on the web in a browser. If that fails too, the problem is your account or network, not the app.
2. Quit Teams fully, from the taskbar or:

Stop-Process -Name ms-teams -Force -ErrorAction SilentlyContinue

Do not kill every msedgewebview2 process. Other apps (new Outlook, Widgets) share it.

3. Install pending Windows updates and restart the PC. Start Teams.
4. Settings > Apps > Installed apps > Microsoft Teams > … > Advanced options. If a Repair button is shown, try it first. It keeps your data.
5. Still blank: quit Teams and clear its cache folder.

Remove-Item "$env:LOCALAPPDATA\Packages\MSTeams_8wekyb3d8bbwe\LocalCache\Microsoft\MSTeams\*" -Recurse -Force

6. Still blank: Advanced options > Reset. This deletes the app's local data and settings and signs you out.
7. Check that Microsoft Edge WebView2 Runtime is listed under Installed apps. If it is missing or broken, reinstall it with Microsoft's Evergreen installer.
8. Third-party antivirus or DLP software? Make sure ms-teams.exe, ms-teamsupdate.exe, msteams_autostarter.exe and msedgewebview2.exe are excluded.
9. Verify: Teams starts from the Start menu, asks you to sign in and loads your chats.

%appdata%\Microsoft\Teams is classic Teams. Deleting it does nothing for new Teams.

← Full article, with the diagnosis and the why