You may try first automatic repair.
If not, try to boot:
- Press and hold the power button until the tablet turns off.
- Press and hold the following buttons (in sequence) to boot into the BIOS: Power button > Volume Up > Voume Down. When done correctly, it will show you the BIOS screen.
But instead of booting from a “real” Windows bootable CD, you need either:
- A repair disk provided by Lenovo (you need to contact Customer Support for this, but I failed in getting a CD from them).
- A repair disk made previously in Windows 8, by following these steps (you should do this as a pre-emptive measure).