My Gutenberg Blocks editor has disappeared

I am trying to learn enough to change from Classic web pages to Block ones so am creating a nonsense page to try out each block as it is discussed in the tutorial.
Suddenly, the block editor stopped working.
There is neither a blue nor a black plus sign. When I click on a block no toolbar appears and there is nothing in the right hand column, which doesn’t even appear.
I suspect that I simply clicked a button that I shouldn’t have, but I’ve spent several days now to get back the screen showing me the editor results.