Locked out of SQL Server

Our active directory was recently migrated to a new domain, and because of this, I could not log in to SQL Server with my new user account. Problem is, I also forgot my sa password, so I could not add my new user account.

This is where single user mode comes in. You restart SQL Server in single-user mode, add the new user or reset the sa password, and then restart it in normal mode.

What do you need to do to start it in single user mode?

Open SQL Server Configuration Manager and click SQL Server Services
Right-click on the instance and click Properties.
Click the Advanced tab, and type the startup parameters into the Startup Parameters box (the parameters are semi-colon delimited, so add -m; to the front.
Click OK
Restart the database engine.

Remember to remove the single user startup option and restart the database after resetting the password or adding the user!

Reference: MSDN