When I float my div to the left, it disappers, why?

I have this div, when I float it to the left, it disappears completely, why?

see here

Here is original CSS code (on wordpress platform):

}
.archive #primary,
.search #primary,
.blog #primary {
    max-width: 900px;
    margin: 0;
    padding: 0;

The div loads, then disappears when I add this: float:left;

I am trying to get the sidebar to sit on the right while the content sits on the left, that’s why I am adding this code.