Are you the DBA for the instance? If not you need to get with the DBA to resolve.

How do you know that the newly created user has anything to do with you not being able to access the instance now? The two seem unrelated. If you can take the instance down you can restart it to gain access from a command line:

net stop mssqlservr

net start mssqlservr -f -m

This should start the instance in single-user mode with minimal configuration. From there you can connect to the instance with sqlcmd and ensure that you're admin account has rights on the instance. Once done you can stop and restart SQL Server and try accessing it again.