undefined field

overwrote my server files with badly designed files.  My backup files are several years old so they are not much help other than reconstructing them.  While working on the DW managed site,  I was prompted what to do with a undefined field.  I choose leave as "unchanged" and lost my content to all files linked to the template.  Is there a means of re-defining the field to reclaim my text?   I hate to start over, however, I learned a valuable lesson.  Thanks.