In between the panes of your double pane windows, there is a gas. This works as an insulator so that the cold outside won’t affect the temperature inside. To keep the gas inside and to block out moisture, the windows are sealed with a silica desiccant. This seal can fail over time, or it could get damaged.
This is due to movement of your windows when the gas inside is heated up and cooled down so that it expands and contracts.
When this seal fails, you will see condensation build up inside of your windows. This will cause your windows to appear foggy or misty. Unfortunately, there is no easy fix for this problem. Most likely, you will need to replace your windows since there is no way to repair the window in a way that will restore it to its original performance. Until the problem is fixed, your windows will no longer work as a good insulator.