A decline to give feedback will send a notification to the participant that you have chosen not to be a respondent.
You will need to contact that person directly and ask if they might re-invite you from their "Manage Respondents" page They may or may not be able to re-invite, depending on whether they have invited someone else in the interim.
If they do re-invite, you will be sent a reminder. Click on the "I would like to help out" button in the reminder email to access your survey response for the participant.