Give us your Graphics Card name.
Try -
right click your desktop and select "Screen Resolution".
Then Click the button "Identify" - hopefully, one screen will show the number "1", the other, number "2".
If so, then change the settings to extend the two monitors
If not - Verify that:
A: your card is able to run dual monitors
B: You have the most up to date driver (to find this, I need your graphics card name).