first check that the two parts of the window (inner and outer panes)
have not parted anywhere around the edge, and letting in moisture.
if all seems still sealed up ok, then look at the top corners on the inside , you will see two small plugs, carefully ease these out , the window should clear after a few days, although if its cold outside it may not clear.
to help it you can use a hairdryer (SET ON COLD) place a small funnel into the plug and put the hairdryer on slow and cold, a small baking funnel will do, you should then see the window start to clear, after it does put the plugs back in.
new plugs are availlable on ebay.
I sometimes find this on one or more of my front windows at this time of year after the early morning sun has been shining on it but the air temperature is low, it mists up. However it does go away of it's own accord if I leave it. If this is not the case, check though as above that it's not due to water ingress due to delamination somewhere, you may see a pool of water in a bottom corner, that shows it's leaking.
Dave.
------------- Give a man a fish and he will eat for a day,
Teach a man to fish and you can get rid of him for a whole weekend.