From 3rd April to 23rd June 2017, the SNCF provides a replacement bus service to operate from Chamonix - Châtelard-Frontière (Swiss border) - Chamonix.
There is an on demand shuttle service between Montroc Le Planet - Argentiere - Montroc Le Planet.
To request the shuttle call +41 27 764 12 71.
Chamonix.net made a test call to this number at 10am on a Friday morning. There was no reply.
You can book your ticket via the SNCF website or at the train stations.
Check out the full timetables for the replacement bus service and the Montroc shuttle on Chamonix.net
The reason for the closure between Chamonix and Châtelard-Frontière (Swiss border) is that a new signalling system (MZ) is being implemented.