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I have used Linux/Unix in the past, however, I am comparatively new to Ubuntu.

We received a new PC with Ubuntu installed. I installed some software and did an upgrade. Since then, I am unable to login.

When I log in, the screen just flashes and then it goes back to the login screen. I cannot log in with the guest account either.

I have found some posts on this forum describing a similar problem and it involves logging to a ttyX, which I am unable to do.

I can fix this by logging in as root, but it is locked by default in Ubuntu.

I have no idea what to do next.

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So, here is the workaround I found:

I managed to access the GRUB screen by hitting the Shift key right after the BIOS started. From there, I chose to restore Ubuntu to factory defaults. Not ideal because I lost the settings and installations I had done before, but still better than installing anew.

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This is a computer that has no data on it, it seems. Just the installation of Ubuntu plus some software. Best solution is to reinstall using 16.04. If it's an older computer or if it has little RAM, make sure you install Ubuntu Mate, and avoid anything with 3D desktops like Gnome3 and Unity. The Mate desktop makes a big difference in speed. You can make this change later if needed.

After installation, make sure you enable the SSH server, so you can login from another computer. You should enable UFW as well, the default firewall for Ubuntu. Enable port 22 to allow incoming SSH traffic. Then test this, so you know it works.

Reinstalling has another bonus. It increases your experience with Ubuntu, and makes sure the system is back to a fresh state. You have more control over what happened to it.

You can choose to use LVM during the installation process. This makes it possible to create snapshots of the system, making it possible to go back to a previous state if things go wrong.

NB: My solution doesn't give a solution to this specific problem. But to me you want your computer to work. Finding a solution that takes hours without any guarantee while reinstalling takes less than an hour - for me it's a no-brainer.

Unless of course the computer has data on it that is important and not backed up. In that case, use an Ubuntu live stick, and copy the data to an external disk and then reinstall.

Of course the same problem could appear after reinstallation. So make sure that SSH server works, and then you can work from there.

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    Hi. Thanks for your help. It looks like from your answer that having SSH working would solve it. We will need remote access, so that is on the list of things to get to work. But I do not see why having SSH working will allow me to work remotely if this happens again: if I cannot login locally, why would I be able to do it remotely? All in all, I am quite disappointed with Ubuntu. I have to stick with it because of my coworkers though. Again, thanks. – Alex Caseiro Jan 2 '17 at 10:58
  • Things can go wrong with Linux just as with Windows. Don't think that Linux or Ubuntu is a flawless system. So if there is no way to fix this problem by the suggestions given in the comments, then reinstallation is the only way. It may result in a system that works for years. Just because you have a problem now, doesn't mean that it will go wrong again. Sorry - no fancy solution, just trial and error. After installation, do all updates, restart, see if it works, then do that software install. If that messed up the display, you know what caused it. – SPRBRN Jan 2 '17 at 11:02
  • And you are probably right about SSH, but it may work! – SPRBRN Jan 2 '17 at 11:05
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    Thanks for your comment. I was not talking about Linux vs Windows, I do not know anything about Windows. I am used to Linux and Unix and if a problem of this scale happens (and of course they do happen), there is always the root login to try. But not in Ubuntu. – Alex Caseiro Jan 2 '17 at 11:05
  • You can create a root user in Ubuntu and later disable it. askubuntu.com/questions/44418/how-to-enable-root-login – SPRBRN Jan 2 '17 at 11:06

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