How many times in a row have you allowed it to try to boot? In my experience when this happens, it will try three or four times and then automatically revert back to Windows 7. Your personal files are intact (unless you HDD has crashed), so it's likely that you can still access them. Something like a live CD / DVD of Ubuntu will let you boot and copy your files if it comes to that, though it's likely you can recover from this without going to that extreme.
As with anything like this, and unfortunately it's too late to say this in you case, always back up data that you need prior to attempting anything like this.