If there are kittens out there and alive - they'll be getting hungry by now and need their mommy. I'd recommend holding onto the one you have and letting her out to look for the others. To get her back to the kitten that you have, you should just be able to allow the little thing to meow and momma should come running. you can even try a little high-pitched meow of your own if the kitten isn't loud enough.

Does she still look pregnant? I remember that when our cat had kittens, she slimmed down imediately. Also, can you get a look at the mother's teats? Kittens will root to a single teat, so if more than one of them looks like its been used, that could also indicate that there are more kittens.

Gosh - good luck and keep us informed.