Recently, I was trapped about which I will now tell.
The shortcut Ctrl+Z in the block editor is used to undo the last actions. For example, if you accidentally deleted the necessary blocks, changed the text, etc., then you can cancel these actions using Ctrl+Z. But in addition to canceling actions, this combination can permanently remove all your blocks! I did not find a way after that to restore them back.
Deletion of all blocks occurs when the action list is empty. For example, you opened a project and did not change anything in the block editor. Ctrl+Z and all blocks from the project are deleted. Another case. You made two changes to the block editor, but accidentally pressed Ctrl+Z the third time - all the blocks are deleted.