Toolbar drop down menu opens on the other screen

When I click a heading on the toolbar - eg File - the menu drops down on my other screen, not underneath the heading, which makes it very difficult to click on the correct menu item
Example:
1. Click File on the toolbar in order to Save As new file
2. Drop down menu appears but on the other screen, not underneath File.
3. Turning off the other screen doesn't help, as the menu isn't then visible Screenshot of drop down menu on second screen.docx