User account doesnt appear anymore

I was working on my computer using Ubuntu 12.04 LTS. Then I wanted to restart my machine because I had a bizzar error when trying to use sudo command. but, when i wanted to Turn Off or just Restart my comp the screens went black, then turn back on and I was still on the "Choose account" screen. So, I decided to turn off my computer by holding the power button and when I try to put the comp back on, all lights went on then 2 sec later everything shut down.... blablabla pass the details, I finaly manage to turn on my computer and when i come to the "supposed choose account" screen it only show the gray box with "Ubuntu" written inside... Try turning off and on again... now it show Ubuntu 12.04 LTS. I retry to turn it on, i had the option to chose "Other..." so I chose this, i enter my supposed account name and password, and the cursor keep loading. I turn it off and on again, and no more "Other..." option it still show "Ubuntu 12.04 LTS". So if everyone know what I could do without formatting (it's my work's computer) it would be very nice.

(sent from Ubuntu Live CD !! :D ) ..

push

Did you do an upgrade or a fresh install of Ubuntu 12.04?
So only the Ubuntu 12.04 LTS logo appears on boot?
Have you tried switching to text console? Ctrl+Alt+F1?
Try to login from the text console and switch back to GUI mode by pressing Ctrl+Alt+F7

I did a fresh install, i was able to login via text console