Vostro won't start - black screen plus continuous beeping

Hi, 

My Dell Vostro 3560 rebooted after installing the latest updates for Windows but when it restarted it showed flashing buttons on the keyboard and then only a black screen with a slow continuous beep.

The beep sounds once every three seconds and just keeps repeating continuously.

After googling for a while, I found the advice to remove the battery, unplug the laptop, hold the power button down for 60sec then plug in the power again and try to reboot. 

This initially worked. But I decided to try and figure out what had caused the problem by rebooting again with the battery removed. 

The same error occurred but this time I couldn't get it to boot by doing the process described above. Every time I tried to reboot it showed the flashing three buttons on the keyboard, then a black screen with a continuous beep. I do hear the fan startup and it sounds like the harddrive starts working. 

Some more googling, and I learned you could start ePSA diagnostics by holding down the function key while pressing the power button to start the laptop.

So I did this and the screen started up and showed the Dell loading screen. It then ran the ePSA diagnostics but that showed that everything was ok. After ePSA the only option is to reboot so I did that and again the same problem: black screen with beeping. So I rebooted again holding down the function key and when the Dell screen was shown and I was able to press F12 and go into the boot options. After looking at the bios, I chose to close the bios and continue booting, and windows loaded ok. 

I'm a bit scared to try rebooting again. But I would like to figure out what the problem is. Any suggestions on what tests I can run?

I don't understand why the Dell screen shows when I hold down the function key and press power, but it the screen doesn't even turn on if I press the power button on its own. 

Could it be a bios setting? or the harddrive?

Any suggestions would be most welcome.

ThanksB