Temporary Solution
When you start your pc keep Left Shift
pressed. it will show you the grub
Permanent solution
If you get to start ubuntu by the temp soln then to make boot permanently show the grub-
Open startup manager(install it if it isnt installed from sofware center)
Change the value for timeout- pref 5 - 10 secs
Default OS - Whatever you want

Rest leave everything as it(you can edit that if you want to)
if that does not work install boot-repair
via ppa
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install boot-repair
This worked for me should work for you too