There are two ways to get grub back.
While running Ubuntu via Pendrive, install boot-repair, open up a terminal and type the following:
sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get update
sudo apt-get install -y boot-repair && boot-repair
boot-repair will automatically launch, if is doesn't, launch it via Dash. Select recommended repair and boot-repair will take care of repairing and getting back grub for you. Reboot.
While running Ubuntu via Pendrive, you can mount the root partition into which you installed Ubuntu. To mount the partition, you can either mount it via Places in Nautilus or via terminal, by typing sudo mount /media/label_or_name_of_root_partition. For example, to mount the Ubuntu root partition labeled as ubuntu, do this:
sudo mount /media/ubuntu
sudo mount /mnt/ubuntu
After mounting the root partition of Ubuntu, reinstall grub by specifying the root Ubuntu partition and hard drive:
sudo grub-install --root-directory=/media/ubuntu /dev/sda
Do not include the partition number on
/dev/sda because grub will be installed on the mbr of the hard disk not on a partition.
After the installation, you should see something like this:
Installation finished. No error reported.
If the time comes and you want to completely remove Windows, you can use GParted to delete the Windows partition and increase the Ubuntu partition with the space gained. There are several tutorials you can find on the web or here to use GParted. This particular post would help you remove windows so Ubuntu fills the hard drive and this Ubuntu Community page remove Windows