There may be a number of possible reasons why your Microsoft Teams won't open. Here are some possible solutions:
1. Make sure you are logged in to the correct 365 account.
2. Make sure you are using a browser that supports Teams. (Note - Safari does not currently support Teams without having to make adjustments to the browser's settings.) You are encouraged to use Google 'Chrome' for your browser. You can download Chrome here.
3. Make sure you have an active connection to the Internet.
Using Teams in Safari
Note: If you want to use Safari, but find that you can't open Teams ... go to the following website to learn how to adjust your settings in Safari so that you can open Teams.