I had Ubuntu 14.04 and Windows 10 dual boot system for a long time. But when the November update for the Windows 10 was released (also called the Threshold 2 update), I reinstalled Windows 10 once again. This resulted in a fresh installation of Windows 10, but Ubuntu boot entry was gone. There is a method to reinstall the GRUB2 bootloader and repair the Linux boot entries using the Boot Repair utility, but it either requires downloading of the full Linux ISO image or using existing Ubuntu CD/ISO image and then giving some commands after booting from the CD.
If you do not want to go through any of these options and want to add the Linux entry to the boot menu from within Windows itself, then you have a very good option – Neosmart EasyBCD utility. The developer of this utility has made a free version available for everyone’s personal use – it is called EasyBCD Community Edition.
EasyBCD is an advanced tool and offers all kinds of boot management but it is available only to work with .NET 4.0. So you must install .NET 4.x before you can start using this tool. It is very easy to add a new entry using EasyBCD. To add a new entry using EasyBCD, you have to click on Add New Entry, select Linux/BSD, select the GRUB2 as the bootloader and leave all the other settings to their default values. Finally you can click on the Add Entry button and it is done.
After this when you can reboot, you would see a boot menu allowing you to choose from Windows and the Linux. You can later change the names of the boot entries using the EasyBCD once again, but it is not really required.
You can download EasyBCD from http://neosmart.net/EasyBCD/ .