During normal use of Teams, you may encounter various issues with it. Those issues can usually be resolved by restarting Teams. However, Teams doesn't actually close when you hit the X button on top of the window. It merely minimizes itself.
Here's a short list of problems that can be fixed by restarting Teams: Freezing, Calls can't be made or are not coming through, Teams is laggy, Can't talk during meetings, and Teams being signed out.
1. Close out your Teams window using the X button on the top right. Again, this doesn't actually close Teams itself. If your Teams is frozen, skip to step 5.
2. Look in the icon tray on the bottom right of your screen for the Teams icon. If you have multiple monitors, you may need to check the icon tray on all of them. Teams will appear on the one that's set as your primary screen.
3. Right click on the Teams icon, and select "Quit". Teams will then fully close.
4. Find Teams in the start menu and re-open it. This should resolve most of Teams issues.
5. If your Teams is frozen, you'll need to close it with the task manager. You can either hit the key comb of Ctrl+Alt+Del, or right click on the start menu and choose Task Manager.
6. Once in the task manager, select Microsoft Teams, and then select End Task. See step 4 to re-open Teams from the Start Menu.