Look down to the bottom of the window and check that the 'PDF viewer' button is grayed out. If it is not, then click it to make the PDF window appear.
Make sure that you also click on 'Options' in the top menu and then click 'configure Texmaker' to open a dialogue box. In the box check that 'Built-in viewer' and 'Embed' are both selected.
Click on 'Options' in the top menu and then click 'configure Texmaker' to open a dialogue box. In the box click 'External viewer'.