Ubuntu Log in becomes black?

[POUYA]

I have installed Ubuntu 10.04, but sometimes when i want to log in, monitor becomes black and does nothing so i have to restart the computer.
I used boot able USB to install My system.

Any one knows why this happens?

[Athar]

This isn't a tech support forum, so you should ask on the official Ubuntu forums - you'll find more people who might know what the problem is there.
I suppose there could be a problem with your graphics driver, so installing a different one might be worth a try.