Is this a CRT monitor on a desktop system ?
If so it may well be that finding an auto set up option (or if it doesn't have one manually adjusting the screen position on the menu/with the controls will fix this) It could be that that last machine to run the monitor at whatever resolution ubuntu starts X with had it at a different refresh and now when ubuntu starts part of the screen is now not visible.
If you have an auto set up option (most screens made in the last 10 years) then wait until you get to the point where the buttons are off screen before pressing it.
TFT's on a analogue VGA connection can also potentially suffer this, but usually they autoadjust by themselves as it is far easier for a TFT to determine that not all pixels are getting fired than it is for a CRT to determine that a scan fills the screen.