If a participant already has a submission in your study then it won't appear in their available studies list.
If for some reason they need to retake/revisit your study, then:
First, make sure you can find their submission in "View submissions". Then, you can send them a direct link to your study via our internal messaging system. By direct link, we mean your Study URL (e.g. https://yourstudy.eu.qualtrics.com/jfe/form/SV_067ESu2P7goO8SN).
Note that participants may need to submit a second response using an incognito/private window on their browser, as some survey platforms prevent duplicate submissions.
When they've completed the study you can use their original submission to reward them.