Unable to boot Windows 8 after Ubuntu installation

My solution to fix the problem is as follows:

Go to bios menu by selecting system setup. Press right arrow to change tabs until you come across secure boot. if it is enabled then disable it. If this works then you might want to disable fast startup as well once you're in Windows.

However disabling fast startup is completely optional. Windows will boot properly even if it is enabled. fast startup should be disabled for those who use files on both Windows and Ubuntu. Or else you might get an Unable to mount the drive error.

Try preparing a USB flash drive or CD-R with my rEFInd boot manager. (Download links for both types of media appear on the download page I've just referenced.) Boot from it. Chances are it will give you two Windows icons with different descriptions. If you're able to boot to Windows from rEFInd using any icon, try installing rEFInd in Ubuntu using the Debian package file. If rEFInd doesn't work or if it works partially but has its own problems, post back with details. (You might want to edit your original question, especially if the problem is complex.) Note that you can adjust the entries that rEFInd shows by cleaning out unwanted boot loaders (files with .efi extensions) from the /boot/efi/EFI directory tree (in Linux) or by editing /boot/efi/EFI/refind/refind.conf and uncommenting and adjusting the dont_scan_files and/or dont_scan_dirs options.