Reboot the box, stop at the loader prompt, type "show" and look for any em tunables you may have had set, for each of those, run "unset xxxxxx" where xxxxxx is the variable name of the tunable, when they're all unset, boot and see if it works.
Odds are that you either had an em tunable in /boot/loader.conf or /boot/loader.conf.local that either had no effect before and is honored now, or that is no longer necessary.
If it boots OK, edit those files and remove any em tunables and see if it reboots OK on its own the next time.