Plesk provides the option to power users (resellers and admins) to hide the sidebar so the user can focus in on the content they're working with. It's often helpful when using the File Manager and editing code. But sometimes if you forget that you've hidden the sidebar or perhaps a window resize has hidden it for you, you don't know how to get it back!

Hover your cursor near the far left side of the window and a small tab with an arrow in it should appear, approximately vertically centred in the browser window. Click the tab and your sidebar will re-appear.