In the NextGen Kanban board, there is an option to Clear the Done issues available on the top of the Done Column. In case the user accidentally clears the issues, there is no direct option to revert this activity. One workaround is to change the status of the issue and drag it to the last column. However, this can be an issue if there are a very high number of issues to be reverted.

Workaround

1) Use Bulk update option or Automation to transition the issues to old status.
2) Drag the issue across the board to the last column.