You probably mean that you forgot your digital certificate password and I presume that this is a self-signed certificate that you yourself created, not a certificate that you received from a Certificate Authority (CA). If this is true then you can create a new self-signed digital certificate (Digital ID in Acrobat-speak) the same way you created the first one: In Acrobat XI it is Edit->Preferences->Signatures->Identities&Trusted Certificates->More. Then click on "Add ID" button and follow the prompts. You can create as many self-signed certificates with the same credentials (name, email, etc.) as you wish. You'd have to figure out how to distinguish them, though.