Here is some info on fixing a corrupted UEFI Partition:
Firstly, boot from a Windows 8 recovery drive (CD/DVD/USB). If you don't have a recovery drive (and this is likely because most new machines don't come with one), then you will either have to install the drive into another Windows 8 machine or obtain a Windows 8 recovery drive, or boot from the recovery partition of the boot drive (If this option works at this point). Don't even bother with the automated recovery process, it will not work as it cannot find a Windows partition.
- You will need to choose the language and time settings.
- Choose 'Repair Your Computer'.
- Choose 'Troubleshooting'.
- Choose "Advanced Options'.
- Next choose 'Command Prompt'.
Next, we're going to use the DISKPART tool to verify that the UEFI partition has a drive letter assignment.
Enter 'DISKPART' (Enter)
(In this test case we removed all other drives except the boot HDD and DVD, so we know the disk 0 is our boot HDD)
DISKPART> sel disk 0
Disk 0 is now the selected disk.
DISKPART> list vol
There was no drive letter assignment to our UEFI partition (volume 3) so we need to assign a drive letter.
DISKPART> sel vol 3
Volume 3 is the selected volume.
1). If you know that you have a extra drive letter available use it here, otherwise skip to 2.
DISKPART> assign letter=E:
2). If you are unsure what the drive chain is or if you don't know what drive letters are avalable use:
If all went well, DiskPart successfully assigned the drive letter or mount point. You can verify that a drive letter was assigned by "DISKPART> list vol" again.
Exit DiskPart tool (Enter EXIT)
Next, we need to change to the boot folder on the UEFI volume.
Change to the UEFI volume into the boot folder. Use drive E: here or whatever drive letter was assigned above.
cd /d E:\Boot\
Now we will need to enter three command lines to repair the BCD store.
For MBR drives: (Drives < 1TB)
- bootrec /fixboot
- bootrec /fixmbr
- bootrec /rebuildmbr
For GPT drives: (Drives > 1TB)
- bootrec /fixboot
- bootrec /rebuildbcd
That's it, if all went well the BCD store should be repaired, and the system will once again boot.