I can't access the Time Machine by clicking - I get a big black box over the screen.

As above. I seem to get to the T. Machine data by opening the external Drive holding T.M. data, and opening the relevant folder - I found the lost file that way. But the timeline on the RHS of the screen, which IS visible when I select the Time Machine icon is non-clickable, irresponsive except that it does enlarge as I scan down the timeline. The rest of my screen - except for an inch of the timeline, is solid black. I don't know when this issue developed - I haven't tried to access the T.M. for years.

Currently Big Sur 11.6.1