When I boot my laptop which has ubuntu 12.04 and Windows 7 I get a message "Invalid system disc Please replace the disc,then press any key" If on this command I press random keys my grub menu appears and my booting to ubuntu 12.04 takes place without any glitch. Also I initially installed 11.10 but upgraded to 12.04 when it was released. Also, no such problem ever occured when I was on 11.10.
Please open BIOS and see the boot priority. In boot priority you have selected a device that is not present in your system. Thats why the error message is appearing. Also you might have connected a portable hard disk or kept cd inside the cd driver and in boot priority that device is set as first boot device.
The problem is with your configuration in BIOS. Its not a problem of ubuntu.