What version of Moodle? What addon plugins installed?
One might keep the Essentials theme (wonder if Essentials has an update availalbe that needs to be applied?), by editing the config.php file and adding a line like this:
$CFG->theme="clean";
No need to restart anything, just hit site, login as admin and see if one can get to admin area.
Essentials theme could require a lot of setup time which if removed might cause some additional work to get back what one had ... 600+ settings?!!!!
Do check plugins to see if there is an update to Essentials ... if so ... acquire.
And, yes, do what Howard suggested about checking for the existence of that file.
'spirit of sharing', Ken