A pending contest will be locked if you do not select a winner 14 days after the contest ends. To select a winner, the contest has to be unlocked, which will open it for another 14 days before it is locked again:
Go to your contest's page.
Click Unlock my contest on the top part of the page.
Click Yes on the pop-up that will appear to proceed, and pay for the $5 unlock fee.
The prize for locked Guaranteed contests will be distributed among the qualified entries if a winner is not selected.