What To Do if You Miss TGV

Make sure your phone has battery and data. If your TGV is a "TGV Inoui", cancel your ticket at least 30 minutes before scheduled departure. This will get you a refund, minus 15 euros.
If your TGV is a "Ouigo", you cannot cancel at such short notice, alas.

You can exchange the ticket for a different train up to 30 minutes before the original train departs. For example. if you have a ticket for a 9:00am train which you can’t make, contact SNCF by 8:30 am at the latest to change your ticket. Be sure to have a printed copy or screen shot of the ticket data ready to go when you land in Paris.
www.SNCF-connect.com

I wrote "cancel" because "cancel" and "exchange" follow similar mechanics, but of course you can exchange rather than cancel!
But if you are pressed for time, cancelling is quicker by a few minutes.

I understand. But how, at CDG do you contact sncf? In person? By phone?

If you download the app SNCF Connect, you can both purchase and cancel using it. I usually turn on my phone the moment the we're on the ground taxiing.

This is generally why it's not advisable to pre-purchase reserved seats on fast trains that connect with incoming flights. If you haven't bought the tickets yet, I would just buy them when you arrive at the airport. There are train stations at CDG, although there may be a long queue at times - https://www.about-paris.com/train-stations-cdg-airport.html .

As an earlier reply mentioned, you could also use the SNCF app - https://apps.apple.com/us/app/sncf-connect-trains-trips/id343889987 (there's an Android version too). I'm not sure whether WiFi is readily available in all parts of the airport so you may need cellular data access.

This is generally why it's not advisable to pre-purchase reserved seats on fast trains that connect with incoming flights. If you haven't bought the tickets yet, I would just buy them when you arrive at the airport. There are train stations at CDG, although there may be a long queue at times - https://www.about-paris.com/train-stations-cdg-airport.html .

I used to agree with this, but increasingly, trains can and will get 100% full at peak and even non-peak times. And TGVs do not accept standing passengers... so, booking is essential!
To manage bookings, the SNCF Connect app is the way to go - as long as you book your tickets thru SNCF Connect! If you use Trainline or Rail Europe (reputable ticket resellers, sometimes more user-friendly than SNCF), use their channels to cancel as well. If you use another travel agent...reconsider. International data plans can be expensive, but it will be cheaper than losing a ticket altogether.

Thank you all so much (!) for the excellent advice about what to do at CDG in the event of a missed TGV, and especially balso for the amazing detailed directions through CDG to the trains, with pictures of the signs along the way!

I agree with the need to commit for advance tickets. Better to hustle through CDG in the attempt to make the train, I reasoned, than to wait to buy and discover that the train is full. Damn the torpedos, full speed ahead!