You should still be able to use whatever password you were using for admin user before making the change. Server user id is not used for restricting access to administrative console (see What is security property 'Server user identity' used for in Websphere Application Server?).
Nonetheless, since you can't access admin console at the moment (I'm assuming you mean that admin colsole login page shows up but you can't get admin user authenticated due to wrong password), first thing I'd suggest is turning of global security, so that you can use admin console to fix your settings and re-enable security. Once security is turned off, you won't be asked for a password during admin console login. Changing security settings in WebSphere Application Server with wsadmin technote gives step by step instructions.
If you are not sure about what password you have used for admin user, you must reset the password in the user repository you have configured (for example if you've configured LDAP, you must reset password at LDAP), before enabling security. If you are using a federated repository (this is default if you've not changed it), most probably admin user resides in file based repository, which is the first member of federated repository. In this case, you can just follow steps at How to reset the administrator's password in the file registry technote to reset your password. Otherwise you can try alternative documented at Updating your WAS administrator password (the link is from Lotus documentation but still is applicable).
