I enabled AHCI mode for SATA (and virtualisation) in my BIOS and rebooted to Windows 8. This failed, so I switched it back. I realise now that something like this is required to change to AHCI mode.
After switching back I am stuck at the "Please wait" screen forever. This happens after selecting Windows 8 from the graphical boot manager and before the login screen. Sometimes there seems to be a small amount of activity judging by the HDD light and sometimes quite a lot. However mostly very little. I've tried leaving it overnight and throughout the day with no progress seemingly being made. Suspend and resume work properly.
Using various advanced boot options has also made no progress: automatic repair fails, safe mode fails, command prompt succeeds but SFC fails. There is no pending.xml. I've also tried using DISM to remove any pending operations, which fails. I have no system restore point. I've tried automatic repair from the DVD also. I've tried re-enabling AHCI mode after following these instructions.
UPDATE: I have just found that booting straight to the Windows disk (BIOS order) gives an error about \Boot\BCD (code: 0xc000000f) being missing or corrupted. Not convinced this is the case, as normally (via GRUB) the bootmgr is functional. Appears to be boot disk ordering issue, having used bcdboot to force boot files onto the drive, it boots again to "Please wait".
This is a multi-boot machine. The Windows XP installation and Ubuntu still work fine.
I would really prefer not to lose all applications and settings with a refresh install. Is there any way to debug this issue further?
UPDATE: out of desperation - and after several more hours trying fail-safe BIOS settings, DISM variations etc. - I tried the "Refresh your PC" option. It also fails with message "There was a problem refreshing your PC". It appears that Windows 8 provides absolutely no information whatsoever about what is wrong, and none of the supplied tools function properly to recover.