I just installed Ubuntu 12.04 64bit, and I cannot start it.

I tried twice. The first time, I used a CD I burned. (The CD works for my another computer, so it is OK.) After wubi did something in Windows, it rebooted. Then the screen was stuck where there is an ubuntu logo at the center and 5 dots below. I waited for more than half hour, it was still there and nothing happened.

The second time, I installed 11.10, then upgrade to 12.04 in a the newly install 11.10. BTW, the 11.10 did start. After upgrading, I rebooted the computer, it was stuck at the same place.

Any one can help me on this? The computer is AMD Phenom II X4 965 + ATI RADEON HD4870. Thanks a lot!