When you vote the app generates some ephemeral voting keys for you. These are forgotten when you close the tab, so you're not able to validate your vote after that point.
However, you can vote twice. Only your most recent ballot will be counted, but this will let you go through and use the "Verify Ballot" feature for the second ballot. Make sure you copy down the details on the "Verify Ballot" page - they are enough to re-make the ballot exactly the same, even if you close the tab.
For other issues, please email support@secure.vote.