WD 40 and tap with a Rubber hammer. If not, have a large 6'9 West Indian mate called Rennie throw you through it. That's unstuck Now BruV!! Bit of a headache afterwards,
There's two schools of thought on WD 40. I've heard a lot of people say it eats rubber, but the company says it's safe for rubber but not good for polycarbonate or polystyrene. Personally I prefer CRC for protection and Vaseline or penetrating oil like Marvel Mystery Oil or Kroll oil for lubrication and breaking things free. But again it really matters what is stuck. Is it the window against the frame, the hinges or the dogs, or are we talking about a slider. Is it metal against metal or plastic. With no other info the best advice I can give is put some Kroll oil on it and let it sit and work in. Do it for a few days and then start working it.