Looking at the time tables for RER C, it looks as though trains stop running in the late morning (11:42) from Versailles to Paris? Is this true, and if so, what public transport is used to get back to Paris?
There are construction works affecting RER C but not between Versailles Rive Gauche and most of the popular RER C stations within Paris (such as Saint Michel Notre Dame).
RER C train from Versailles to Paris should be running throughout the day at a frequency of about every 15 minutes.
Is there a particular day you're looking to travel from Versailles to Paris that appears to be affected by closures?
Whenever there is a full interruption of train service, a replacement bus is provided in between the last operating station and the continuing station after the construction/maintenance works.