If as you say your system is running at defaults, and you have not changed any hardware (or BIOS) in the day before the problem started, then I would suggest you attempt to use the rescue disc's that would have come with your OEM system.
If you do not have system discs and you are unable to rebuild the MBR then booting via a live CD/USB will allow you to enter the hard drive and save your data. It does seem like you are going to be doing a fresh install of OS.
I like both Slax and Puppy easy to use and simple to download and create the live USB OS.
To help us help you - please provide as much information about your system and the problem as possible.