deleting a header cannot be undone. I deleted a header to delete an empty page, the header was deleted on all pages, the empty page vanished. The problem is that I didn't know what was in the header but the document needed a certain format, this meant that all changes to the document had to be lost in order to regain the header ...
Unfortunately this is a known bug. There is a dialog that warns you that you won't be able to undo the deletion *** This bug has been marked as a duplicate of bug 46561 ***
(In reply to comment #0) > [...] > The problem is that I didn't know what was in the header but the document > needed a certain format, this meant that all changes to the document had to > be lost in order to regain the header ... If you do not want to lost your changes, you should save it with another name (menu File > Save as...). Then you can reopen the original file with its header. Best regards. JBF