I have just been thru this.... had to change all my bookings to get to Les Houches
on 22/12 due to no trains. Suggest you go onto 'Trainline' site as I did and go thru/work back from 'arrivals' at your proposed destination to see what is still running there, arrival times and from where..... you will find some trains & most buses are running and may be your best option. I ended up rebooking TGV from Avignon
to Lyon and then getting a bus from Lyon to Chamonix
. Will arrive about same time tomorrow but unfortunately cost me a nights accomodation due to changes. If you have received a new 'rebooking' & wish to cancel it check the (your) email address they sent to you on the email as it may have been changed on the new booking - mine had been changed to some random 30+ encoded new email address and unless you copy & paste this address when trying to retrieve the booking, the site won't recognise you....... I assume a sneaky attempt to stop people cancelling bookings & getting refunds! Good luck.... hope this helps.