You can only have one review in progress at a time. Once a review period is completed, your account admins should archive all reviews to allow new self-assessments to be started.
If they haven't done so, you can archive the review yourself. Go to Reviews > Your review and click 'Review actions > Archive this review'. You'll then see the 'Your review' link in the left-hand navbar change to 'Start self-assessment' - click here to start a new review for yourself.
Managers: you can do the same for your direct reports. Open their review from the Direct reports tab, then follow the same steps to archive their review for them.
Note: if you're not seeing the option to archive your own review, this means your account administrators have activated a setting that allows only admins to archive items. Please get in touch with your HR Team / admins to archive the review for you.