By locking the database and issuing a single write, all readers are starved. No read operations can go through.
Because of this, a new shell will start but cannot be used – it must be doing some read-lock requiring operation. Ostensibly, this is due to the shell attempting to read but being unable to do so due to the waiting write starving any reader attempt.
Issuing "mongo --eval 'db.fsyncUnlock()'" will unlock the database, allow writes through, and unfreeze the shell.