Q:

Suddenly unable to start 16.04, even after reinstall

I'm having a bad day. This morning my 16.04 refused to start. I go through the Grub loader, choose Ubuntu (I have dual boot with Windows 10). I reach the Ubuntu splash screen with the colored dots, and nothing happens. I have a laptop (Dell Inspiron) with an ATI video card(Radeon 8730M), and I see often errors related to Radeon, but things still work. Today, nothing. I tried a few things like restarting with nomodeset in the grub configuration, or failsafe graphics in recovery mode, I always end up with a black screen. I never get to the login screen. Ctrl-Alt-F1 does not bring me to a terminal.
I tried a bit everything in recovery mode (fsck, grub, fix packages, etc.) to no avail. Things were complicated by the fact I couldn't get the wifi to work in the recovery shell (dhclient wlan0 never returns), so I had to plug in a network cable to run apt-get upgrade and such.
I burned a CD with the 16.04 ISO. I started with it, it gave me an option to reinstall Ubuntu on top of my install. Did that, it gave me one error about one of my package that it couldn't reinstall, to do it manually afterwards. Upon restart, I go straight into emergency mode. The last message tells me that ISO-8859-1 is not supported on /dev/sda1, my boot partition. Running depmod tells me that /etc/modules/4.4.0.22 does not exist. Yes, only 4.4.0.21 exists.
So if I start the standard ubuntu it tries the 4.4.0.22 kernel and doesn't find the modules, so goes straight to emergency mode. If I go through the advanced grub menu and choose 4.4.0.21, I go back to the black screen I had this morning. A day lost and nothing accomplished... Things were working fine yesterday, and the laptop shut down correctly last night.
Thanks for any pointer.

A:

OK, poring over the log I saw a GTK error. I had installed manually GTK 3.20, I was sure I had restarted since but apparently not. Uninstalling GTK 3.20 to go back to 3.18 solved my issue. Thanks to everybody who chimed in with suggestions.

