How can I ask the respondents to complete the survey they did partially ?
This largely depends on how you have initially distributed your survey.
If you sent the default link to all respondents, you will need to be able to identify their responses via Results > Individual Responses. If you are able to identify which Partial responses belong to who, you can email the respondents their ‘edit links’ as described here:
If you distributed your survey via an Email Campaign, you can set up Reminder Messages which automatically only send to those respondents who have Not Started or are Partial:
Hope this helps!