Fix WSL - cannot access '/mnt/c': Input/output error

I hit a problem with my WSL Ubuntu system, where I can't ls the C:\ drive from within my WSL Ubuntu Linux system...

mpenning@wsl$ ls /mnt/cls: cannot access '/mnt/c': Input/output errormpenning@wsl$

How do I fix this so I can see the contents of C:\ from inside my WSL Ubuntu system?