I was trying to change workflow, it was saved, but all boards had disappeared.

[Lena]

"You can't see them here because your current project is not configured to use boards. "

How to configure project to use boards?

[Nic Brough -Adaptavist-]

You've set your workflow up to do something the business boards can't handle.

The link you've got in that screenshot probably goes to https://support.atlassian.com/jira-core-cloud/docs/work-with-boards-in-business-projects/ which explains what workflow configurations break and hence hide business boards.