Once the time runs out, your assessment is automatically submitted. Click OK in the time-out message that pops up and the Assessment Results page opens. Just like if you'd clicked the Complete button, you'll also receive an email and a notification in Touchstone with your results.
If you want to retake the assessment, contact your administrator. They can restart the assessment so it'll show up again on your "My Assessments" page and you can complete the assessment again.
Remember that the time out clock starts counting down as soon as you open the assessment. It's counting down the time you have to complete the whole assessment from start to finish, not just one section or page.