Dell XPS (L521X) boots and stops in Dell LOGO

I have Dell XPS (L521X), which I bought in 2012. It was working fine and I wanted to reinstall the OS. I formatted the HDD and installed it but all of a sudden my laptop shut down. And now everytime I want to restart my laptop, it will boot and stops at Dell Logo. I want to go to BIOS by pressing F2 key and it won't work and it won't let me choose Boot Options by pressing F12 key either. It stays there forever and my screen goes black and shuts down. I have installed extra memory module in the laptop. Is it the problem of the fault memory in the system. It was working fine but all of a sudden. I tried replacing only one of the memory module and its working fine. Earlier it was making two beep sound, and I had to fix the memory and there is no beep sound now. But the problem is still there, it boots up to the Dell Logo and stays there.