Cannot boot without USB stick after successful install [duplicate]

I installed Ubuntu yesterday via a bootable USB drive, created with unetbootin. Everything went very well, but now the computer does not boot unless the USB drive is plugged in. If it's not, then when the computer boots, the boot sequence screen just appears and if I select the hard drive, it will just reboot and go back to this boot sequence screen, until I plug in the USB drive.

Any ideas? Thanks a lot for your help!

UPDATE: Having some valuable data on the machine, I would prefer a solution that doesn't involve reinstalling, but from the first replies, I guess this may be what I have to do eventually. Thanks!

GM

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EDIT: output of sudo fdisk -l below

Disk /dev/sda: 750.2 GB, 750156374016 bytes
255 heads, 63 sectors/track, 91201 cylinders, total 1465149168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x000742dc

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1            2048  1449474047   724736000   83  Linux
/dev/sda2      1449476094  1465147391     7835649    5  Extended
Partition 2 does not start on physical sector boundary.
/dev/sda5      1449476096  1465147391     7835648   82  Linux swap / Solaris

Disk /dev/sdb: 4009 MB, 4009754624 bytes
255 heads, 63 sectors/track, 487 cylinders, total 7831552 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xc3072e18

   Device Boot      Start         End      Blocks   Id  System
        /dev/sdb1   *        8064     7831551     3911744    b  W95 FAT32

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EDIT2 bootinfo output: http://paste.ubuntu.com/1006423

It seems that Ubuntu has been installed in the USB instead of in the hard drive. This maybe related to a bug that is described (in Spanish) in the following address:

http://www.ubuntu-es.org/node/168068

If this is the bug that affects you, it seems that can be solved using the following command:

sudo grub-install /dev/sda && sudo update-grub

Just make sure that this is the issue that affects you before using this workaround!

I have solved this problem thanks to help on another forum.

I used boot-repair, ticking "reinstall grub" as well as "purge and reinstall". When I had to reboot, the screen froze and I had to use the power button. But now it boots fine on the hard drive. I didn't have to make another partition.

Thanks everybody for your help.