Paragraph number is always 0.0.0

I have an existing old document (ca 25 years) which I keep updating. It has a number of chapters and within these are various numbered paragraph lists of instructions.  In recent upgrades, these automatic lists have corrupted to 0.0.0.  I cannot find any way to recreate the numbered list, except deleting all the paragraph codes then manually typing 1.<tab> , 2.<tab> etc. for all the lists in this large document.  Somewhere there must be a code which has been upset by an upgrade and gone rogue .  My current version is 18.0.0.200

Any suggestions what and where it might be?

Thank you