Boot from ubuntu 14.04 Live CD/Boot the emergency disk and open a text-mode shell.
Type gdisk /dev/sda (change /dev/sda to whatever is appropriate to access your hard disk, if necessary).
The program is likely to complain that it's found both MBR and GPT data, and will ask which to use. It doesn't matter which you tell it to use.
At the Command prompt, type x to enter the experts' menu.
At the Expert command prompt, type z to zap (destroy) the GPT data.
Type y in response to the confirmation about destroying the GPT.
Type n in response to the query about blanking the MBR. Caution: If you answer y here, you'll destroy Windows partition(s)!
# parted -l --------- To Check the Partition table Structure
# apt-get install gdisk
# gdisk /dev/sda
...
Command (? for help):x
Expert Command (? for help):z
About to wipeout GPT on /dev/sda Proceed? (Y/N):y
"GPT Data Structures Destroyed"
Blank out MBR (Y/N):n
"MBR is unchanged"
#
# reboot