If you had an OS installed and it no longer boots all of a sudden, it could indicate a failed drive, in which case you would no longer be able to retreve data off the drive unless you take it to one of those expensive data retrival companies.
Have you made sure the proper boot deivce is in fact selected in BIOS as well?,
If the OS is just corrupted and most data is still intact you can try booting up a live CD of one of the many Linux distros (i use Fedora 17) and try retriving your data that way.