try this if it does not work try below one.
Try entering your BIOS menu by entering f10 or f9 or f12 or delete key OR any other key that you can try.
Once there, there will be an option for boot order and in that after selecting OS boot there will be options for choosing windows OR ubuntu as first option.
In my BIOS for rearranging I have to press f6 and f5 yours may be different.
Make a live Ubuntu USB or CD and boot to it.
Select "try Ubuntu without installing" Once there, open terminal and execute the following:
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair && boot-repair
After installing, open it by executing
boot-repair and select recommended repair then follow on screen instructions.
After booting for first time execute
sudo update-grub to add all entries and you are good to go