Sending, receiving, and organizing email in Outlook.com
Rules can disappear or appear missing if they were deleted, if you changed between Outlook.com/new Outlook/classic Outlook, if the rules were client-side rules created in classic Outlook, or if an account/sync issue was using a third-party app.
Regarding restoring them: Unfortunately, Outlook.com does not provide a “deleted rules” recovery folder like deleted emails. Microsoft states that rules import/export is available only in classic Outlook, not new Outlook or Outlook on the web.