It's a monitor design quirk causing that. You got the message because Ubuntu read the info your monitor fed back to your videocard and somehow understood it could run the monitor at a higher or lower frequency than the monitor can really work with; say, 120Hz rather than 70 or 55Hz refresh rates.
If you don't even get the first logo menu, try hitting either down-arrow, then enter, or F4. Those correspond to menu choices to boot in failsafe graphics mode or get the menu in such a mode.
Answers & Comments
It's a monitor design quirk causing that.
You got the message because Ubuntu read the info your monitor fed back to your videocard and somehow understood it could run the monitor at a higher or lower frequency than the monitor can really work with; say, 120Hz rather than 70 or 55Hz refresh rates.
If you don't even get the first logo menu, try hitting either down-arrow, then enter, or F4. Those correspond to menu choices to boot in failsafe graphics mode or get the menu in such a mode.