In the Create case section, select Account and billing support, and enter all the required details. If your need is urgent, choose the Phone contact method. An AWS Support agent contacts you by phone to help you reopen your account. If you closed your account less than 90 days ago, then you can sign in to your account and file a support case.
https://aws.amazon.com/premiumsupport/knowledge-center/reopen-aws-account/