Ok,
using the recovery usb you have now boot back into the recovery console. Using this guide to open the command prompt app
When the computer boots off the recovery media, pick the Troubleshoot option
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Then choose Advanced Options
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and head on over to the Command Prompt.
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If you can get the command prompt app working type:
bootrec.exe /rebuildbcd
Press enter.
If the above doesn't work then your going to need an install disk/USB of Windows 8.1 so you can boot from that and repair the bootloader.