Try running Microsoft Teams as an administrator. Right-click the Teams shortcut and select "Run as administrator."
Try resetting the desktop app by copy this: %appdata%\Microsoft\Teams and paste it via Start and hit enter. Delete all content in the location and start Teams again.
Cached data and cookies can sometimes interfere with the loading of web applications. Clear your browser's cache and cookies, especially if you are using the web version of Microsoft Teams.
Try a different browser.
Ensure that your firewall or security software is not blocking Microsoft Teams. Check your firewall settings to allow Teams to access the internet.