How to reinstall grub

Try this

mkdir /mnt/root
mount /dev/sda7 /mnt/root
mount /dev/sda5 /mnt/root/boot
mount --bind /dev /mnt/root/dev
mount --bind /proc /mnt/root/proc
mount --bind /sys /mnt/root/sys

This will create a chroot environment and now you need to run chroot and create configuration file for grub

chroot /mnt/root
/usr/sbin/update-grub2
reboot

Now it should works

First boot from live Ubuntu CD or Live Ubuntu flash drive and connect to internet. After that go to terminal by Ctrl+Alt+T and just use the command below,

sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo sed 's/trusty/saucy/g' -i /etc/apt/sources.list.d/yannubuntu-boot-repair-trusty.list
sudo apt-get update
sudo apt-get install -y boot-repair && (boot-repair &)

This actually fixes any problems with boot. If it still does not work for you, just leave the error report in the add comment section. You will find I have attached a link, it has screenshots and clear instruction. Just go to the link if something still not clear to you. Have Fun.

Reference link: Installing Boot-Repair in Ubuntu