boot - Broken 14.04 LTS while upgrading to 16.04 - Ask Ubuntu

i tried upgrade, errors occured. tail of output

import process completed. done done. processing triggers libgdk-pixbuf2.0-0:amd64 (2.32.2-1ubuntu1.2) ... processing triggers ureadahead (0.100.0-19) ... processing triggers dbus (1.10.6-1ubuntu3.1) ... processing triggers resolvconf (1.78ubuntu2) ... processing triggers vlc-nox (2.2.2-5) ... processing triggers bamfdaemon (0.5.3~bzr0+16.04.20160824-0ubuntu1) ... rebuilding /usr/share/applications/bamf-2.index... errors encountered while processing:  emacs24  emacs error in function:    not install upgrades   upgrade has aborted. system in unusable state.  recovery run (dpkg --configure -a).   setting emacs24 (24.5+1-6ubuntu1) ... install cmake-data emacs24 install/cmake-data: byte-compiling emacs24 emacs24: relocation error: /usr/lib/x86_64-linux-gnu/libboost_system.so.1.58.0: symbol _znst7__cxx1112basic_stringicst11char_traitsicesaiceed1ev, version glibcxx_3.4.21 not defined in file libstdc++.so.6 link time reference install emacsen-common emacs24 emacsen-common: handling install of emacsen flavor emacs24 emacs24: relocation error: /usr/lib/x86_64-linux-gnu/libboost_system.so.1.58.0: symbol _znst7__cxx1112basic_stringicst11char_traitsicesaiceed1ev, version glibcxx_3.4.21 not defined in file libstdc++.so.6 link time reference error: install script emacsen-common package failed dpkg: error processing package emacs24 (--configure):  subprocess installed post-installation script returned error exit status 1 dpkg: dependency problems prevent configuration of emacs:  emacs depends on emacs24 | emacs24-lucid | emacs24-nox; however:   package emacs24 not configured yet.   package emacs24-lucid not installed.   package emacs24-nox not installed.  dpkg: error processing package emacs (--configure):  dependency problems - leaving unconfigured errors encountered while processing:  emacs24  emacs  upgrade complete   upgrade has completed there errors during upgrade  process.   continue please press [enter] 

i rebooted, , system not boot. posts message

/dev/sda6: clean, 645654/20442624 files, 21700419/113769472 blocks 

and keeps flashing mildly screen. idea on how can recover boot?

i know 1 option reinstall. subject of install ubuntu iso... cannot install grub

you try booting older kernel grub menu. holding shift key while booting should give grub boot menu.

hopefully see newer kernel installed part of upgrade , older kernel version boot into.

if older kernel option boot it. can try fix broken packages with

sudo apt-get -f install 

from previous experience give , clean install if things don't go smoothly when upgrading. in fact when went 14.04 16.04 choose clean install option. may able perceive , fix boot problem find other issues exist , may spend more time worth fixing them.

worst case (and easiest option) suggest using live cd boot , files off onto usb drive clean install.