The easiest way to solve dual boot problems is to use Boot-Repair (IMHO).
What is Boot-Repair?
Boot-Repair is a simple tool to repair frequent boot issues you may encounter in Ubuntu like when you can't boot Ubuntu after installing Windows or another Linux distribution, or when you can't boot Windows after installing Ubuntu, or when GRUB is not displayed anymore, some upgrade breaks GRUB, etc.
Installation and use:
- Boot into Ubuntu Live CD or USB
Hit Alt+Ctrl+T to open terminal and run following commands:
sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get update
sudo apt-get install -y boot-repair && boot-repair
Once done Boot-Repair will open with two options choose the first option (Recommended repair).
When repair is finished, Save the URL that appeared, Then reboot.
If the repair didn't succeed, Share the URL on askubuntu or any other help forum for more help.