Is your question, "How do I find her?" Do you not know where she is? Did she move or is she traveling around, or don't you know? Not sure what "took off" means. If you don't even know where she is, then you have to hire a private investigator - but that's expensive and Canada is a big country! And what will you do with the info once you get it? If she's not talking to you, what's your plan?
Canada is easier for Americans to get in and out of without a passport than are other countries. So maybe she had her ID and his birth certificate?
Or is your question, "How does a child travel back to the US without his mother?" It matters where you have any type of legal connection (custody, guardianship). Did she take him out of your custody? Or does she have custody and you just want to be able to see him? How old is he and is he old enough to travel unaccompanied once the ID question is settled? My mother took my 11 year old son to Bermuda with just a notarized letter from my husband and me authorizing her to travel, seek medical attention, etc., and she had his birth certificate and medical insurance info. But that was before 9/11 so things have changed.
If I were you, and if there's no legal/custody issue at play here and a good relationship with your daughter, I'd either call the consulate/State Dept. (check the on line info first), or call my Congressman's office and ask what they know. They know the government, often they can grease the wheels for you, and that's their job.
ETA: Here, I just looked it up for you. Americans can go to Canada with a photo ID (license) and proof of citizenship (birth certificate, voter ID card). Children under 16 only need proof of citizenship (birth certificate if born in US). Here's the website: https://travel.state.gov/content/passports/en/country/can...