Try editing the page and then editing the documents web part to show the toolbar.
- Click the gear icon in the upper right of the screen and select Edit Page.
- Select the web part.
- Click the arrow next to the checkbox for the web part and select Edit Web Part.
- Select Show Toolbar in the Toolbar Type drop-down.
- Click Apply and then check if the toolbar shows up.
- Click OK to stop editing the web part.
- Save your changes to the page and stop editing it.
As I mentioned in my comment, the toolbar may appear different in this view (at least it does for me), but you can change the views using the toolbar now.
This happened to me a while back and I thought that it was because I had added a script editor to the page with a snippet to show 5 views instead of the default 3.