Error 15 is an error code in Grub Legacy; File not Found which is usually referring to the kernel or initrd file. Grub4Dos still basically uses terminology from Grub Legacy. Mount the partition from another Linux system if you have any installed or from a Live DVD/flash drive and see if the files are where they should be.
Is this the only OS on the Computer?
As asked above, why Grub4Dos? Grub2, which comes with Ubuntu is a much better bootloader.
Also, you make reference to windows 10. Do you or did you have windows 10 on this machine and if so, was it an EFI install?
You will probably need to post more information such as partition and boot files. Maybe download and run the bootinfoscript and post the output here. Or you can go to the site below and download boot repair and burn it to a CD and boot with it. If you select the option to CreateBootInfo Summary, it will give you a link you can post here which provides detailed information on your system.
https://help.ubuntu.com/community/Boot-Repair