Wild guess is that your new mothedoard BIOS support's UEFI.
Whit EFI / UEFI you have booting point at motherboard memory and
you get that easyly lost :
-After doing FRESH install you must also
UPDATE starting location to motherboard memory ,you might forget to
do that or installation script dose not note that!
-updateing BIOS will clear out EFI startup info as well.
-editing EFI BIOS SETTINGS can cause same !!!
At Ubuntu 12.4 precise simply BOOT computer WHIT UEFI whit your
LIVE-CD, RESCUE DISK/STICK/USB and go repair mode.
You must get your self loged in / chrooted to your hard disk
and then reinstall EFI booting information to system memory
as well update HARDDISK information as well.
You need startup code (at my system /sda) at starup hardisk loacated at /boot/efi
and whit my configuration mounting goes like this:
mount /dev/sda /boot/efi
So you got mounted booting part needed by grub-efi program to system
Just then to update motherboard UEFI memory settings as well
harddisk whit grub-efi run:
apt-get install --reinstall grub-efi-amd64
apt-get install --reinstall grub-efi
This should do all needed work (memory, harddisk) things and then just reboot!!!
MORE: http://www.thinkwiki.org/wiki/UEFI_Firmware https://help.ubuntu.com/community/UEFIBooting