I see this bite people irregularly over the years: if your default shell on login can’t run, what do you do?  I’ve seen it happen because of a missing /usr/lib, and it can happen with out-of-date library references, too.   There’s several different ways to deal with it:

• Run a shell that can’t have this problem, like /bin/tcsh (the root default).
• Or, rebuild in single-user mode from the console.
• Or, perform the bullet-proof upgrade.

That last one may be useful if your dports setup gets mangled, somehow – though ‘pkg upgrade’ has always worked for me.