Hello Leonie, thank you very much for your reply. I used to have a "Library" button in the sidebar, under which was every photo that's stored on my Mac, and also a separate "Import" button. I could click on the "Import" button to view imported photos and then drag them from there into their appropriate Albums. Perhaps this format has now changed, as you suggest, as I no longer have "Library" or "Import" buttons in the sidebar.
I've just imported some new photos from my iPhone and found that, after a lengthy delay, these photos have been added to what may have replaced the "Library" function, although it no longer has a name or a button - every photo, historic and new, now seems to be in 1 very large folder. I don't know how to get back into this un-labelled section holding all my photos, other than when I'm importing new ones, as there seems to be no button to access it (eg, if I find that a photo has gone missing from an Album and I want to drag it back in from what used to be "Imports" into the Album again). I hope what I'm trying to explain makes some sense...
I've also checked the Filter, as you suggest, and it's set to "All items".