At this point the best thing is probably just to keep an eye on her. If she vomits at all in the next 24-48 hours, it is critical that you take her to the emergency vet immediately. But hopefully you (and she!) were lucky and she managed to poop out everything she swallowed.
You can have x-rays taken for peace of mind if you want. X-rays won't show the ribbon itself because cloth is not radiodense (this is why we don't have to take off our clothes when we have x-rays done), but it will show if the intestines seem to be out of their usual place, as would be the case if there is a string tangled in them. But you can also get "false positives" from this kind of x-ray because the intestines naturally shift as food passes through them, and "false negatives" because it's possible that the string isn't tangled at that particular moment but could get stuck later on. So rather than spending $200 on x-rays that will be inconclusive, it's probably better to just keep an eye on her. If she's still fine by, say, Saturday morning, then it's a pretty safe bet that she will be fine.
To make sure your bases are covered, though, take a few minutes now to look up the phone number of the nearest 24 hour emergency vet clinic and print out Mapquest driving directions from your house to get there. That way if the worst happens you won't have to waste time looking up the information and you can just head out the door. (Actually, this is smart for everyone since we usually don't know ahead of time when an emergency will happen! You can keep the Mapquest printout in the glove compartment of your car so it's always right where you need it.)
For the future, please never ever leave your cat unsupervised with a ribbon or string or rubber band or anything else she could easily swallow. If a cat swallows a bit of string he can't spit it out and will just keep swallowing until the string is gone or he manages to bite it off. Sometimes he vomits or poops it out without a problem, but often it gets stuck in the intestines. The only treatment at that time is emergency surgery to cut the ribbon out of the intestines. This is obviously no fun for Kitty, and it will be no fun for you to pay the vet bill which will be well over $1000! But without the surgery the cat will in all likelihood die, and sadly I have known a couple of cats to die even with surgery. It's very serious stuff.
Playtime with strings and ribbons and yarn and the like should always be supervised very closely. Take the toy away if you have to leave for even a moment. If you do catch your cat in the act of swallowing a string, you can pull very gently on the part that is still outside the cat's mouth. This should trigger his gag reflex and you should be able to pull it the rest of the way out without much trouble. If you can't do this, head straight to the nearest emergency hospital. They can induce vomiting there and the string will come up before it has a chance to tangle in the intestines. Your cat will get to go home with you in an hour or so. A $100-$200 emergency visit to induce vomiting is much better than having to go through surgery. If you notice string hanging out of your cat's anus, do the same thing as if he is in the process of swallowing the string. Pull very, very gently and stop if you feel any resistance. It should come right out, but if it doesn't then wait and see if it comes out with the next bowel movement. If that doesn't happen and the cat seems to be straining in the litterbox or otherwise uncomfortable, then again it's off to the emergency vet.
Anyhow, I know this is a crazy long post. Point is that cats and ribbons can be a really tragic combination. So just make sure you supervise your cat's playtime so she stays safe.