I cannot see nor change the BibTeX entries any longer. Usually I saw them when I clicked on the line main table
I guess you can see the list of entries in the entry table, right?
Do you mean that, when you double-click on one line of the table, nothing happens? (i.e. the entry editor does not opens)
Which version of JabRef are you using?
I am using JabRef-4.3.1 since several years. All worked until yesterday
So, it should not be too bad.
My guess is that the entry editor is only 1 pixel high. So, it is opened at the lower part of the window, but very small. By moving the mouse near the bottom of the window, you should be able to grab its upper border and give it a decent size.
It occurred to me before…
By the way, in JabRef 5.1 (the current version), this has been solved (a minimum decent height has been given to the entry editor)
JabRef is helping you on a regular basis? Please, find your way of giving back.