I have installed the Ubuntu 12.04 using a bootable USB. The installation went fine but after the installation there was just a blank screen with the cursor blinking and Ubuntu wasn't starting. Maybe that was an installation error but when I booted again from my USB surprisingly Ubuntu started working. Even if I remove the USB after the initial boot, Ubuntu keeps working fine. Do I have to boot from my USB each time I restart my computer?
Grub, Ubuntu's bootloader, was installed on the flash drive, and not on your internal hard drive, as it should have been.
Boot with the flash drive plugged in. Remove it once Ubuntu is booted up.
Run the following commands from a Terminal:
Click on the "Recommended repair" button, and reboot.
Boot-Repair installed Grub to your internal hard drive. This allows your computer to boot without depending on the bootloader that was installed on the flash drive.
As SirCarlo says on his answer, this is caused for not installing mbr on the internel drive.
I prefer this solution myself.
Note: If you have more than one hard drive, including USB drive, the system assigns letter