For this situation you have to connect to database in Single-User mode.
Starting SQL Server in single-user mode enables any member of the computer’s local Administrators group to connect to the instance of SQL Server as a member of the sysadmin fixed server role.
Here you can find step-by-step instruction to do this.
In short you must start the sqlserver instance with parameters -m, after start Sql Server Management Studio with windows authentication.
Now you are a sysadmin, assign the sysadmin role to your user, exit and remove the -m parameter and restart sql server.