Dell Latitude E6520 Laptop Unable to Boot Windows 7

Any help appreciated on this problem:

Yesterday, while surfing the web, some of my programs suddenly closed automatically (Internet Explorer 9 and Maplestory). Before I could pull up Task Manager, the system was logging me off. In a moment of panic, I pressed and held the power button to shut down. I waited a few seconds before booting my laptop, but to my disappointment, the screen never could no longer get past the screen with a flashing "_" character at the top left of the display. I've reset the Setup to default and run a Diagnostic without any issues, but still can not get Windows 7 to start. F5 and F8 are not doing anything useful. 

Some things I noticed:

Dell power-up process works fine; after its completion, I can see some text including the word "Initializing" for less than a second before the screen switches to the unresponsive flashing "_" screen. 

Also, at the flashing "_" screen, F5 seems to trigger a loud beep, whereas holding Fan and tapping F8 does not make that sound.

I recently got a Windows notice that recommended I install another Windows update to combat new viruses. I did not install that update yet (that was the day of the final shutdown).