Can not edit the block in Drupal 8

I am new to Drupal. I started a month ago with Drupal 8. Everything works fine. I've had some own custom blocks. Then I added javascript to a block. Immediately after, editing the block or other content was no longer possible. I can edit nodes, if I open them, there are tabs. But my own block is not possible to edit. If I delete this block, I can edit any other content. If I play it again, the edit button (pen) is missing. How can I edit my own block again?