Blegh, it seems that every time I go away for an extended period and come back to graduate work in the HPC lab here, something profound gets broken with my user account. Last time I had half of my software suddenly stop working right, and had to log in as the root user on any PC to get anything done. This time it's just completely hosed.
This is why regular consumers don't use Linux. It is just unbelievably maliciously unhelpful if you do anything to it.
Or, even if you don't do anything to it. In this case, I just tried to log in. I get this wonderfully unhelpful error message: "Unable to start session." Great. How did that happen and why? I thought it was my password, but no, after resetting it nothing changed. So, I google it and find some helpful looking commands, which I ran over SSH (which for some reason works...).
Now the fun starts. I reboot it again and try to log in and... blank desktop. Hey, that's improvement. But what do I do now? Google isn't much help this time, since all of the answers are about reinstalling the desktop manager package, and the problem only exists for my user account. I don't want to risk trashing everything else when it's almost certainly just my account that's messed up.
So, I figure that maybe this PC has been messed with and I need to reconfigure some other things. We have a nice document here on how to set up new PCs for the lab, so I go through all of the hilariously verbose Linux commands to set up the NIS server, add DNS configuration, NFS mounts, network interfaces... the list goes on. I figure that at best nothing will happen and at worst... nothing will happen. It was set up this way before, after all.
So, I reboot. Oops, now it hangs during the startup screen complaining that some NFS mounts aren't available. Reboot into recovery mode. Spend 1 hour trying to figure out how to remount the / filesystem as read and write so I can fix the NFS mounts (it just hangs if I run any recovery menu option that does this for me). Find out the NFS mounts are fine. Spend 1 hour trying to figure out how to get DNS working again. Okay, got that fixed. Reboot. Still hangs while booting. Reboot into recovery mode. Go to console. Manually start up the network interfaces and mount the NFS file systems. Go back to Unity. Hey, I can get to the login prompt now. Log in and... blank desktop.
I've gone through all of the logs I can find anywhere and found that my user account somehow is over its disk quota. Dropping it to half the quota didn't fix anything. I tried deleting various profile files and rebooting. No good.
Oh, and hey, look at that, now I can't log in over SSH or anything because my password is now incorrect (it's not).
And to top it all off, the magical wizards who have encyclopedic knowledge of Linux and how to fix this are nowhere to be seen today.
Great, just wasted another day that could have been put toward my research.
I hate being a grad student.
...I kinda hate working with Linux too. You almost need a graduate degree in Linux to work with Linux. It seems that most people online troubleshooting Linux boil it down to taking wild guesses and tossing commands at you that are probably for a different distro 15 versions ago and have nothing to do with what's actually wrong.