\Ubuntu\winboot\wubildr.mbr missing or contains errors

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http://askubuntu.com – I installed Ubuntu 12.04 version using WUBI when my laptop was running windows 7 which I upgraded to windows 8.1 preview. Everything was working well until I upgraded again to windows 8.1 retail when it was released a few weeks ago. After selecting Ubuntu during boot option, boot manager showed the below error message. "The os couldn't be loaded because the file \Ubuntu\winboot\wubildr.mbr is missing or contains error". I have tried the Boot-Repair option twice now and chkdsk /r /f /x (every option that is supposed to fix errors) but to no avail. Please help me because my thesis is on there an (HowTos)