If it is the first time you are facing this problem and you were able
to successfully mount it before ,then it is possibly due to unclean
shutdown of windows. If this is the case then fix your file system by
using windows volume error checking.
First try ntfs-config software and if it doesn't work ,then consider
the second solution
But, if you are not able to mount ntfs from the very start , then
consider doing manual setting in /etc/fstab file (http://
ubuntuforums.org/showthread.php?t=802699).
If you are not sure how to do that , then post the output of following
commands :-
1)sudo fdisk -l
Then , i will provide you step-by-step guide.