Why is sidebar not showing up on my iMac Photos app?

I have a number of Apple devices and the main one I use - desktop iMac is no longer showing the sidebar that shows Photos / Collections / Albums / Shared Albums on the left of the screen. All I have visible is the entire photos library pictures. I want to drag and drop some photos into Albums but cannot do that.

I cannot locate anywhere that tells me how to show the sidebar.

I have an iBook and that shows the sidebar. I have checked the settings in view and both Macs have the same settings but the iMac is not showing the sidebar.

Any help would be greatly appreciated - it is kind of driving me nuts!

[Re-Titled by Moderator]