We often (too often) have birds fly into our windows, sometimes they hit amazingly hard, and 90% of the time they end up just fine and fly away. We don't intervene any more then going out and checking on them occasionally, but it is probably a good idea to make sure they're not in plain sight. Perhaps you can camoflague it by placing some branches or leaves around it. To tell you the truth, I think the only fatality we ever had was a chickadee who unfortunately chose the coldest day of the year to smash into our window (I think it was about -10 degrees and it froze). We actually did try to intervene with that one and it didn't work. Most likely your cardinal will be fine. Do you have plants or something thing that looks like a bright flower in your window? Sometimes that attracts birds, especially in the winter.