(redirected) Alienware X51 R1 continuously beeps at startup

Hi, 

Recently, my X51 R1 suddenly restarted in the middle of a program (windows explorer) then it beeped continuously during boot and no image was displayed on screen.  I have searched the net for similar cases and have found out that it may be because of the motherboard.  I have been using this for 1 year and 8 months now. The only hardware upgrade I did was changing the stock GTX 555 OEM to a Palit GTX 660Ti reference card (on December 2013).

In order to deal with the beeping issue, I reseated my Video card then waited for several minutes before turning on the machine (by the way, I put my X51 on a vertical position).  The same thing happened again. 

After that, I reseated my RAM and Video card then waited for several minutes.  I turned on the machine while on a horizontal position, and it booted without errors.

What can be the cause of the beeping issue? If there is something wrong with my motherboard, it should not work anymore, correct?  Is it beneficial to put the X51 on a horizontal position?

I hope someone can shed light on this.  Thanks in advance!