How do I go about fixing this window that won't pull shut, either temporarily or permanently? So much cold air is leaking in
Look closely at another window. Move the lever to get an idea what should be happening. From the outside, with the window open, look for a removable plate that covers the mechanism. Open a good one to see how it works, then open the bad one to see what is not working. If it can't be fixed easily, remove the whole mechanism and go to local window suppliers. It should be removeable. Good luck!