Thank you all. I have never encouted this problem before. But I remember there are several threads about this problem: when booting up, they just see a "L" or some "L"s on the screen and the system halts. I tried to search for these threads but can not find the keyword.
So, I outlined a more clearer topic and *hope* to close this problem and make it convinent for other newbies like me
Again, thank you all.