BUILTIN\Administrators accidently removed
Start the SQL Server service in single-user mode (by using the -m startup parameter).  When you do this, all members of the local administrators group will have sysadmin rights.  Provided you are a member of that group, you should be able to do whatever sysadmin actions you need.

Take a look at Database Engine Service Startup Options for documentation.

Edit:  To start SQL Server in Single-User Mode, please see this documentation.