Hello, and thank you for using this service.I will help you now.
Do you have your display settings to extended desktop or mirror display?
I sent an offer for phone support, because it makes things a lot faster and easier when dealing with the chromebook. If you don't want phone support, please try the steps below and let me know results:
To adjust your external display:
From here, you can identify displays or adjust their location.
Are you able to get into Display settings? if so, please tell me what you see. thanks.
this is why I was suggesting phone support, because the layout of the settings can change across different books and displays.
What you are looking for is a setting to MIRROR the display and/or a setting to turn OFF unified desktop mode. If you notice your cursor on the external monitor when you move it well off the main screen, this will confirm that the problem you are having is that your displays are extended and not mirrored. Also, understand you can and many people do use their monitors like this, as it gives you the ability to put different windows on the two screens, which is what most people want (as opposed to mirror).