Hello Bar is covering a header in most cases because the option to Pushes page down was not enabled in the Style section. Enabling this option should resolve your issue.
However, sometimes Hello Bar can still cover your header even with Pushes page down option enabled. This means that your header is using a fixed CSS position for the navigation bar on your website, this is the same position that Hello Bar uses. Your header can only be pushed down if the top navigation bar is not set to fixed or absolute. You can either adjust your websites code, change Hello Bar to appear at the bottom of the page or choose to use a different Hello Bar style such as a slider, modal or takeover.