The fogging means that the seal between the two panes of glass has broken and moisture has gotten in between them. The only real concern at this point other than the look is that the window has lost its energy efficiency. Depending on the window, you may be able to replace just the glass, but more than likely you should replace the entire window itself. By replacing the entire window, you will have a product with a warranty and won’t have to deal with this in the future. If you just replace the glass, the odds are it will have again in the future.
– Jerry Fleenor, Window & Door Expert at Bordner