If this happens, check your BIOS setup device boot order. It is possible that some device has been listed before your real boot device.
In my case, my boot SSD drives had been moved below ALL PCI SCSI and there are other drives in the machine which are not bootable causing this problem for me.
It might actually not be hanged, but instead but you might have been kicked into an initramfs shell but it is not displaying on the screen.
Try to boot in recovery mode or boot on a install CD or USB stick in rescue mode. Edit /etc/default/grub:
vi /etc/default/grub
Set the following:
GRUB_TERMINAL=console
GRUB_GFXPAYLOAD_LINUX=text
If the GRUB timeout is very short it might be a good idea to set it a bit higher so you have a chance to make a selection:
GRUB_TIMEOUT=10
Save and then execute:
update-grub
Then reboot the machine and now you may see the emergency shell you are kicked into. Now you can investigate your boot problems further.