You asked what happened 😉
To revert use Undo (CTRL+Z) a few times. If that doesn’t take you back far enough:
Go to Front View and with Vertex Select and in Wireframe Mode select one half and delete it and the select the rest and extrude on X-axis.
Can’t see what your X Mirror is doing though, but the trick should work anyway, just don’t delete the ‘mirror-line’ vertices.
