One of the problems with the new versions of Excel is that you often need to close Excel down completely to start afresh. In earlier versions you could close the workbooks in such a way that you end up with a grey screen (Excel open but no open workbooks) and you could open a new workbook from there. Two ways to achieve the same things in Excel 2013 and up i.e. close the workbook but not Excel
The shortcut to achieve this is to use CTRL + W. So with your last workbook open click CTRL + W.
You will end up with this. You can now open a workbook or start a new one (CTRL + N opens a blank workbook).
The Menu option
You can achieve the same thing with the close menu option under the File menu. In the last workbook, instead of clicking the x in the top right (which would close Excel down), click on the File Ribbon and click on the Close option as shown below.
You will get back to the grey screen