In a dual pane window there is a frame of aluminum and desiccant sandwiched between two pieces of glass. there is a seal caulked all the way around the window. When the seal breaks, moisture comes in, and the desiccant no longer works. To fix the issue, the frame needs to be replaced along with the glass. They will either take your window apart and fix it (very unlikely) or recommend that you replace it (which is the smartest choice anyway).