What is the CSM option in BIOS? If I can't boot into Windows any more, is it because CSM is disabled?
Would an ASUS K55 come with this option diabled?
CSM or Compatibility Support Module is something that allows booting in legacy BIOS mode on UEFI systems.
The Compatibility Support Module (CSM) is a component of the UEFI firmware that provides legacy BIOS compatibility by emulating a BIOS environment, allowing legacy operating systems and some option ROMs that do not support UEFI to still be used.
Suffice it to say that if your PC is fairly new and came with Windows pre-installed, CSM would've been disabled by default. You don't need to enable it. It's only needed if you must install an older OS that doesn't support UEFI.
If you've mucked around in the BIOS settings, reset it to defaults and see if your PC boots again. Most BIOSes have a keyboard shortcut to reset to factory default settings. It's F10 on my BIOS.