I tried installing Ubuntu 14.04 once, and it was successful. However, after I booted my computer to a windows 10 usb drive, grub disappeared. I tried to retrieve it several times, but to no avail. I then deleted my Ubuntu partition of my computer to start over. But, when I reinstalled Ubuntu and restarted to finish the installation, neither grub nor Ubuntu showed up, my computer went straight to windows. I need help getting Ubuntu and grub back.
It is possible that you have Fast Boot enabled. This prevents GRUB from booting. Try http://www.windows10update.com/2015/05/windows-10-tutorials-66-how-to-enable-or-disable-fast-startup/.
It is also possible that you need to change your BIOS settings. While your computer is booting, press f2 or esc or delete (depends on manufacturer) and change the boot order so that the Ubuntu boots first.
If neither of those options work, boot into Ubuntu Live and download boot-repair and run it.
Reformat your usb stick or buy another . Re-download image,Ensure the CheckSums and SHASUMS are correct. Partition your disks(if you have 1TB ROM split 50-50 so both OS's have plenty of space). You don't need to format the partition made for Ubuntu all you have to do is make space. Once that is done restart your computer with Ubuntu Image on the USB stick in place and follow the steps. When it asks to run alongside Windows click yes/ok/check and Ubuntu should install.
Here are more detailed steps:http://www.everydaylinuxuser.com/2015/11/how-to-install-ubuntu-linux-alongside.html Along with the link from the 1st answer http://www.windows10update.com/2015/05/windows-10-tutorials-66-how-to-enable-or-disable-fast-startup/
You should be good to go.