unless you changed some settings ,or messed with partitions, or ran some "Optimizer" or maybe your AV program is out of date and blah, blah, blah.... but , if you're sure you didn't do anything , I would pull the HDD and replace it with a new one .
Other than that , you should be able to boot up your repair disc and give it a go or try a system restore to a point before this happened...
Was booting my pc and after some time I received a message "Load boot device". I could not locate my HDD in BIOS at first. BUt later it was visible.. now whenever I try to boot it recognizes my hdd but it can't start windows..and starts up again and displays that annoying message..please help..