I thought there was no way to recover from a mistake a like that, but another friend found the way and gave us a link with the solution.
Basically, we need to stop the SqlService, and restart it in "single user mode" and "minimal config mode"
sqlservr.exe -m -r -s SQLEXPRESS
And the open another command window and run this commands:
osql -E -S .\SQLEXPRESS exec sp_password @new='changeme', @loginame='sa' go alter login sa enable go exit
And that was it. Remember, without the special "-m -r" options, the osql commands will fail.
NOTE: In case your SQL Server is not configured to use Mixed Mode for authentication, you may also need to make some modification to the registry settings.
Basically you need to find the registry entry for the instance:
HKLM\Software\Microsoft\Microsoft SQL Server\Instance Name\MSSQLServer\LoginMode