The magically disappearing fonts

Yesterday I had a whole bunch of fonts, which are installed, that were in my Font List but, today, they are no longer listed. I have installed AND non-installed fonts ticked in the filter and, to me, the weirdest thing is that some of these fonts are used in a drawing I'm currently working on - and they are working just fine. I've never encountered anything at all like this before.

Does anyone happen to know the cure for this malady?

Thank you!