If you have dual boot system ubuntu and windows and you get this error unknown filesystem you would want to recover your windows first priority. You need to start your computer with extranal Ubuntu live CD or USB (if not sure ask I can explain how to create one).
once you run ubuntu then you need to open commandline (Ctrl+Shift + t) you need to be root to access root type:
Then type following commands successively
sudo add-apt-repository ppa:yannubuntu/boot-repair sudo apt-get update && sudo apt-get install -y boot-repair && boot-repair
Once done will open the boot-repair windows with two options choose the first option ( need to be patient it will take time).Restart computer without the CD or USB and see if it fixes the problem if not then run you live ubuntu again and type’gksu boot-repair’ in the commandline. It will popup the window again and choose the second option. Restart and see if it fixes the problem if not follow the link here for further help.