You can try Last Known Good COnfiguration in F8 to see if it will boot Win8. You can also download the WIn8 ISO file to burn to DVD or write to flash stick, boot to Repair.
Another method is given above to run Factory Recovery by tapping the F10 key at boot. If this is how you want to reinstall, try that too.
Otherwise you will need to either order your Vaio Recovery Disks from Sony Tech Support, or do the clean reinstall with the installer ISO I linked you.
Did you get your files copied out?