Another option, which is quick, is this: If you have a "good" (earlier) version of your document with the styles intact, use the "Change styles" menu to save that style.  Then, go into the "bad" version of the document and apply the styles you saved in the good version. This was a 5-minute fix, and worked for me. The changes remained after I closed and re-opened the document. 
There are three settings we need to embed in this field. The first is to tell it what kind of numbering we want to do (in this case, “First, Second, Third”), what case we want to use (upper case, title case, etc.), and a switch to tell Microsoft Word to increment the numbers. Click each of these settings as shown below, being sure to click Add to Field after each one: desktop numbering using word