[Discuss] Serious error in /usr

[Murray Strome]

When I reboot into Ubuntu (16.04) on at least one computer, first it 
will not boot. If I chose Advanced Options, and Upstart I get a message 
saying something like "Serious error in /usr", which is a separate 
partition.

I am given a few options, but if I chose "I" to ignore the error, it 
does boot OK, and I have several options for Windows managers. In the 
past, Icewm was often the best, but right now, Plasma (i.e. KDE) seems 
to be working the best.

What I would like to know is:
How can I find out what the "serious errors" might be, and how can I fix 
them?  I really do not want to have to go through the pain of 
reinstalling Ubuntu if I don't have to do so.

Any help would be greatly appreciated.

Murray