If you had the user's email address set properly (and the smtp configuration set properly), you could click login in the UI and select forgot password, enter the username and email address, and a new password will be sent to you. If you do not have email setup properly, you will need to manually edit the security.xml file, located in (nexus-basedir)/../sonatype-work/nexus/conf/security.xml. First stop nexus, then open the file, and find the <user> object that has a <id> of admin. Then change the <password> to f865b53623b121fd34ee5426c792e5c33af8c227 . This will give the admin user a password of admin123 (the default) and you will again be able to login.
