If you are out of leads you have couple of options:
1.You can Upload new list with leads in your campaign.
2. You can reset your lists and they will be dialable again:
2.1. You need to click on Lists / Reset Leads and to select the campaign and the list that you want to reset.
2.2. Second option to reset list is from Lists menu
For this you need to go View lists, open the list that you want to reset and select Reset List – Yes