The majority of the widgets on the Matrix home page can be dragged around and rearranged, making the home page highly customizable. Widgets can even become hidden or removed from the home page. When this happens, they end up in the Additional widget and need to be dragged back onto the home page:
Go to the Additional widget* and put your mouse cursor over the individual widget you'd like back on the home page (the cursor will turn into a four headed arrow and the widget item will get outlined with a dotted line). Hold down the left mouse button, drag it to an empty spot on the home page and release it:
It will lock into place and once again allow you to use it properly.
* Since the widgets can be moved around, the placement of your Additional widget may be different than the image in this article. You'll have to search all three columns on the Matrix home page to find it.