We need more information as lots of things could be possible. Is this your first
use of Linux? Is this a change or something new, ie. it was working but isn't
now? Has it happened before? What is your hardware, especially video adapter?
What is your setup of Ubuntu, ie. are you running Compiz or anything that might
affect video? What have you done to try to resolve the issue? Can you open a
terminal or Synaptic?