In case of dual boot system, if you install Ubuntu after Windows, then there should not be any problem in booting into both the Operating systems. Both Ubuntu and Windows entry can be found in Ubuntu boot menu. But if you install Windows after Ubuntu, then you can't boot into Ubuntu. There will be no boot menu entry for Ubuntu. Actually Windows boot loader override Ubuntu's boot loader and does not make any entry in boot menu for Ubuntu.
It is a very well knows issue and it can be easily solved by restoring GRUB. This method works for both Ubuntu and Linuxmint. It is 100% tested and working method. To do this you need a bootable Live CD or Pendrive of Ubuntu.
1. First boot into Ubuntu Live CD or Pendrive and Run the System.
2. Then Open Terminal and type the following commands one by one.
sudo fdisk -l
sudo mount /dev/sdaX /mnt
for example if ubuntu is installed in sda2 partition, then replace sdaX with sda2
sudo grub-install –root-directory=/mnt/ /dev/sda
If no error occurs during this process that means GRUB has been restored successfully.
Now reboot and Boot menu entry of Ubuntu and Windows both can be found. This method works for Ubuntu as well as Linuxmint.