By default the Ribbon in Microsoft OneNote collapses to provide extra screen space for taking your notes. If you want the Ribbon to stay expanded to give you quick access to the functions you use most you can pin the Ribbon down.

Bottom Right

With the Ribbon expanded, look in the bottom right hand corner for a tiny pushpin icon. Click on this icon to leave the Ribbon in expanded mode.

Collapse

To give yourself more space to work in OneNote you can go back to the bottom right hand corner of the Ribbon and click on the upside down V icon (caret).