I dual booted Windows 8 and ubuntu 12.04 (64 bit) recently, everything was running fine, no errors except ubuntu crashed in unity 3d mode but worked fine with 2d. Yesterday, I was showing Windows 8 to my cousins (I hardly use Windows 8 anymore) after an hour of use I restarted the computer to boot into ubuntu, but ubuntu didn't boot. I even tried boot-repair using Live USB (I thought it might work after boot repair, but it didn't). What should I do now? GRUB won't even load, the system directly boots into Windows 8. Also, the system had Windows 8 pre-installed.
The problem is actually Windows and not Linux. Microsoft wrote their system to try and hijack the computer as a whole from any other operating systems. Multiple windows updates (I'm guessing you have automatic updates turned on for windows?) will try to kill the GRUB bootloader.
Although there are software ways around this (such as reinstalling Ubuntu), my personal suggestion for avoiding this continually is to add a second internal harddrive to your computer. (This is much better for system stability anyway! One harddrive for each operating system, and one "documents and media" hard drive where you store your files, pictures, movies, etc. This usually ensures that if something dies horribly, you won't be completely lost, especially as any big errors to happen will USUALLY happen on the operating system's harddrive.) Then, you can avoid GRUB and windows overrides entirely but just hitting your computers bios "boot device menu" key when booting and just change which drive you're using.