Why doesn't the Queen have as much power?

Ok, back in the old days, the King and Queen of England had absolute power. When and why did the royalty lose total power? The Queen doesn't do nearly as much as Queens in the past have done, is that her decision or is it that the laws have changed? Is the Prime Minister the one with complete power now? I know that the Queen advises and does charity, but when she pes and Charles takes over, will he do more, is he allowed to do more, could he possibly take most of the work from the PM? Or has it now become a law that the King and/or Queen has limitations? I understand being direct descendants of the originals centuries ago and that's why they get all of the wealth. Could it possibly turn into the positions turn into just a title and that's it? They can do nothing and still be King or Queen? I really hope to have a Brit answer this with lots of knowledge on the subject. I was wondering about all of this after the Jerry Seinfeld comment about the Royal Wedding not really being as important as everyone is making it out to be. I think it is a big deal. William is a descendant to the original Royalty so yes, it is a big deal.