I have just tried to expand my main win8 partition. All went well until a power cut in my area. Anyway, windows booted fine, but I decided to run chkdsk.When I rebooted, it tried to boot both of my windows 8 and 8.1 installations!!! I literally saw 2 windows logos on top of each other. Now nothing works. All i get when I select my OS (when i dont get bsod), is a blank screen. I notice if I put my pc to sleep and wake it up, my mouse appears, but nothing else.

If you have any bootable Win 8 media, you might try doing a startup repair, or if that fails, an actual boot repair...

I'm not real sure how well this process works when you're using a dual boot setup though.

Check this link for tips on how to get to Advanced Options from boot media....

http://pcsupport.about.com/od/windows-8/a/open-advanced-startup-options-windows-8.htm

Here's a command set to run from the .CMD prompt on the installation\repair media....

Anything you boot with should get you to the .CMD, the steps may just vary a bit...
1. Put the Windows installation disc in the disc drive, and then start the computer.
2. Press a key when the message indicating "Press any key to boot from CD or DVD …". appears.
3. Select a language, a time, a currency, and a keyboard or another input method, and then click Next.
4. Click Repair your computer.
5. Click the operating system that you want to repair, and then click Next.
6. In the System Recovery Options dialog box, click Command Prompt.
7. Type BOOTREC /FIXMBR, and then press ENTER.
8. Type BOOTREC /FIXBOOT, and then press ENTER.
9. Type BOOTREC /REBUILDBCD, and then press ENTER.