Ubuntu – Cannot boot to Ubuntu, though installation was successful

First thing first, you need to repair your WIn7 bootloader, and you can visit this page to learn how to do this. Write down on a piece of paper the required commands for fixing both boot and mbr. You'll be using those codes after getting to the Command Prompt screen.

Next you can download Boot Repair iso image as suggested in the previous answer by 'user3005324'. Choose 64 bit iso image if you have installed Ubuntu 64 bit or choose 32 bit iso image if you have Ubuntu 32 bit. I hope Win7 is the same architecture with Ubuntu, I mean you have either Win7 32 bit and Ubuntu 32 bit, or you have Win7 64 bit and Ubuntu 64 bit.

Burn the iso image, and restart your computer with the new Boot Repair CD in tray. You can write Boot Repair iso image on a USB pendrive with Unetbootin tool if you don't have a regular CD to use. Read instructions on how to use Unetbootin in Linux/Ubuntu, they are located at the middle of this page: http://unetbootin.sourceforge.net/ .

After Boot Repair cd or usb pendrive finish loading, you can press the 'Recommended Repair' option, wait to finish and after you get the message 'success' you can reboot and use Win7 and Ubuntu which will be present both in your start-up boot menu.

Disable the hibernation mode and Fast Boot in Windows.
Open command prompt as administrator and execute :

powercfg /h off  

Open the legacy version of the Windows Control Panel (not the modern version).
Select Energy Settings, enable show hidden settings and uncheck Fast Boot.
After having done this - shutdown the computer completely - do NOT reboot.
Boot into the BIOS and select Ubuntu as the default operating system to boot.

Update :

In case there is no Ubuntu entry to be found in the BIOS / UEFI settings,
re-install the GRUB boot loader to your Ubuntu installation in EFI mode.
Boot from the Ubuntu installation media - open a terminal and execute:

sudo mount /dev/sdXXX /mnt
sudo mount /dev/sdXX /mnt/boot/efi
for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done
sudo chroot /mnt
grub-install /dev/sdX
update-grub  

Note:

sdX = disk | sdXX = efi partition | sdXXX = system partition

To identify the partition numbers use GParted - it is included in the Ubuntu installation media.
Boot into BIOS and select Ubuntu in UEFI settings to be the default operating system to boot.