Can no longer drag my mouse to expand a selection

SergioDCQ The dot (bottom corner of the selection) is at the bottom left corner of A5.  This is what it looks like if you have the entire row selected.  

There is a distinction between expanding your selection and dragging that dot (known as the "fill handle").

Dragging the fill handle will copy the cells in the selection to the area where you drag and release. 

Expanding your selection does not require the fill handle.  You can simply click anywhere, hold the mouse button down, and drag to expand your selection.  If you already have more than one cell selected, you can hold down the Shift key and then click-drag with your mouse to expand the selection.