I have a Dell Inspiron laptop that I no longer use much. I had installed Ubuntu 12.04 on a partition. I loved it and decided to remove the Windows partition altogether. However, after doing so, I now cannot boot into Ubuntu, and get a missing bootmgr error message. I have been trying to read through similar posts, but I just can't get it. If anyone could walk me through a fix, I would greatly appreciate it.
I can only assume you used the old Windows boot manager to boot Ubuntu with, that Ubuntu is at least version 10.04 or later and that you actually installed Ubuntu and NOT along side of Windows like what Wubi does.
So if you really "deleted" Windows and now exclusively use Ubuntu then you don't need that old Microsoft boot manager anyway. You don't need it even if you dual boot with Ubuntu either. Problem is, that tiny little 100-Meg NTFS partition is probably still the one and only "boot" partition on the hard drive and still being used - or at least, it was. But the fix is pretty simple. Just change the boot flag on the hard drive from that 100-Meg partition to wherever GRUB was installed which is likely on the one and only ext4 partition (not an NTFS one).
You might also note that this 100-Meg NTFS Windows boot partition is typically hidden from Windows but that it can easily be exposed if you simply assign it a drive letter in Windows. That's not exactly an Ubuntu thing. I only mention it since this hidden 100-Meg Windows "System" partition is where the Windows boot manager lives - it's not on the actual C: drive, which, again I assume you deleted. But who cares? You don't even use Windows any more. Right?
So to change the boot flag to wherever GRUB lives, just boot from a live Ubuntu disk/key/(whatever - you Pixie booter you) and fire up GParted. If you like command lines you could instead use FDISK or even CFDISK but it's nowhere near as pretty. Note that GParted is on the live Ubuntu disk (as well as countless other distros) but is never actually installed when you install Ubuntu. So using GParted, find the partition that has GRUB on it which is typically on the one and only ext4 partition and in a folder called /boot. Use GParted to assign that ext4 partition the boot "flag" and you're done. (Alternatively, as with any partition editor, you can do much destructive things like adjust partitions, create partitions, or even delete them. So be careful!)
In GParted specifically, highlight whatever partition that contains Ubuntu's /boot folder (which is where GRUB lives and is likely identified as /dev/hda1 with an ext4 file type), then select "Partition" from GParted's menu, go to "Manage Flags" and be sure the "boot" box is checked. Exit GParted (or whatever) and reboot from the hard drive that you just fixed.
If you were using the old Windows boot manager (XP to present day) then the boot flag on that old 100-Meg NTFS partition will be set. Again, this 100-Meg partition is unnecessary for GRUB and Ubuntu. That is, unless you did something during the Ubuntu installation that we still don't know about. And assuming there aren't any other problems with GRUB or anything else, which could very well exist, you should now be using GRUB to boot with instead of the old Windows boot manager. (You might also note that the GRUB menu doesn't even show up during the boot process unless it has detected another OS like Windows - or even another Linux OS.)
Don't worry about the boot flag thing either. You can always change it back or even to any other partition on that one physical hard drive without fear of any real damage. However you do need to check your BIOS setting to know what physical hard drive is actually booting! Because if you have more than one physical hard drive attached then THIS may be all you need to check!
Hope it helps.