You might want to apply for an extension to your visa (30 EUR fee), since a cancellation of a flight might be considered an exceptional case (per the regulation (EC) No 810/2009, Visa Code, CHAPTER V, Article 33). See here for more details, a similar question:
Schengen visa extension (
http://www.maxi-pedia.com/forum/index.php?topic=837.msg2284#msg2284). That would be the case for a flight being cancelled after you entered the Schengen country, and that possibly
might be the case for a flight being cancelled after you have already paid for your ticket. In case you have not paid for the ticket yet, and it is just your reservation that has changed, than you are limited by the duration for which your visa was issued. To read about consequences of overstaying Schengen visa, search this forum (many posts on this topic have already been submitted).