First of all, you should make sure that your computer will not overheat again. Overheating is very bad for a computer. I suggest you dust your computer, and if it's a laptop, get a cooling pad.
Do you still have a bootable flash drive with Ubuntu on it? If not, you'll want to create one, using a flash drive, an Ubuntu ISO, and Rufus.
Once you're booted, you'll want to add the repos for and install Boot-Repair, an automatic tool to repair and restore GRUB. If Ubuntu is still intact, it should be able to fix GRUB. If you still can't seem to boot GRUB, then you should check around in your BIOS, if you have a UEFI system then it's possible that each OS has its own boot entry and that you'll need to select Ubuntu or the drive itself instead.
Also, when you have managed to boot into Ubuntu, you should upgrade the OS. 14.10 is no longer supported at all. Run the command
sudo do-release-upgrade to upgrade the OS.
However, if you still can't boot into Ubuntu or GRUB, then it'll probably be easiest to re-install Ubuntu. You should upgrade your Ubuntu version anyway. Copy whatever files that you can access from Ubuntu's partitions and re-install using the 16.04.1 LTS ISO.