First things first, you want to ask... Is it Fixable? 
When it comes to Windows, there are two main parts. The frame and the glass. So identifying the two will allow you to inspect, first the frame and then around the glass. Now houses get old as well as shift. Caulking around the frame may be an option.. If it has big gaps around the frame and/or crooked, you will need a new frame.
Now if your frame has passed inspection, now is time to check the glass.
What can happen is moisture can sit and eat at the glass unit seal and deteriorate it. That leads to moisture inside the glass and it continuously builds up and eventually stays which causes milky looking glass. 
As well, shifting of the frame can break the glass and unnoticeable.