My flat screen has sound but no picture what do I do to fix it??

Are you using an antenna, cable or satellite? If cable or sat, how is it connected to the TV?

If you are using HDMI, that carries picture and sound. If you are using regular RCA cables, make sure they are all in and on the correct inputs.

Also, some TV's let you turn off the speakers in the setup menu, or configure it for external speakers. Check that.

Model numbers help if you need more specifics.