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I did a clean installation of ubuntu 12.04 from a usb stick. After installation, first boot was normal so i logged in installed some apps and lenses and downloaded updates. When I rebooted to install updates after bios screen it said boot manager is missing, press alt+ctrl+del to restart. so how can i repair/install mbr? thanx

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if you put the USB that you used to install into the computer and then restart will Ubuntu boot? if so your boot loader was installed onto the USB instead of the hard disk. Make sure that you have booted into your installation instead of the LiveUSB(it will not ask you if you want to "try Ubuntu" and you will see y the account you created during install if you have booted to the HDD) If you have booted into your install you can fix the bootloader by reading this answer – TrailRider Jul 6 '13 at 1:46
even if you cannot boot into the install you can still fix the MBR if the install was done correctly(as in with no errors being thrown, not your error) by booting into the LiveUSB and then following the directions in the answer. If the install was fine but the boot loader was corrupted this will still work. Note however that if you boot into the liveUSB the boot-repair program will only be installed in the live environment not to the hard disk so it will be gone upon reboot but the repairs will remain to the mbr. (I have used the program myself so I know it works IF the install is good. – TrailRider Jul 6 '13 at 1:51
up vote 0 down vote accepted

The boot manager for Ubuntu, GRUB2, was probably broken when you were installing updates. Try using Boot Repair to fix it. First boot to the Ubuntu live CD. Open a terminal window and run the following code to install and run boot repair:

sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair

Then follow the instructions and options in the program to repair your boot.

You can visit for more information.

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