Hello jorgedacunha,
It can be a several problems.
1. Do you have the latest graphic card drivers.
2. Can you set your windows theme to default and see if it comes back.
3. Is your screen zoomed to a normal 100% some windows graphic card options are between 100 and 150%
If this doesnt work you can set your quick access toolbar below your ribbon this should work at all times
To move the Quick Access toolbar

The Quick Access toolbar can display above or below the ribbon. To change the setting, on the Quick Access toolbar, click Customize Quick Access Toolbar drop-down
Show Below the Ribbon.