When a job has been live on the website for longer than the set number of Live Days (set in admin under Jobs), it will be come expired. You will see a red cross next to the job if it is either un-published or has expired.
To renew it you should click on the job title and then arrive on the overview page. Click on EDIT in the top right of the job advert to update job details.
Once in the edit a job page you should scroll down to the bottom of the job advert page where you will see some an option to renew the job. When you click this and confirm the job will be reposted again for as long as the default Live days setting is.