I've just upgraded my Lenovo laptop from 16.04.LTS to 18.04 LTS and now can't get into Ubuntu (though dual loading with Win10 is OK).
When I get to the login screen, my user id appears, but if I then click on it I get a flash of the next screen (presumably where I'd enter my password) and
Authentication Error appears and the system hangs - it shows the last block check data.
From the login screen (if I don't click on my name) I can use Ctrl and then log in and get to the command line. I've tried to do an update but it seems I don't have an internet connection (though it was working OK before the upgrade) so all of the answers I've looked at relating to similar questions won't work.
I asked the question initially on the Ubuntu Forums and only got one response which was to run
sudo apt remove upstart --purge but which came with the warning that I ran it at my own risk! Again, it seems I might have a problem downloading any replacement packages.
Can somebody help me please?
Added 5 Aug.
I tried the poster's suggestion but to no avail. From a bootable disk I can access the internet using an eternet connection, however, from the root of the installed version I can't. As far as I can tell only my bluetooth is off, wifi should be on but isn't working. If I try to run an update it can't access the ubuntu URL. I was going to reinstall Ubuntu (but don't want to lose exisiting info) but can't work out how to tell it which sda to use as boot, etc. - it just isn't intuitive Perhaps it's time to admit defeat and go back to Windows?