Installing and upgrading help

You must mean upgraded to 3.7.x of Moodle. ;)

Oops ... major theme changes to 3.7.x of Moodle.  boost is a/the fallback theme used by moodle when other in-compatible themes are present.  Thus deleting boost throws the error.  (tip: never delete ... just hide core)

To get back to a point where you can see the newly upgraded moodle, add the following line to config.php of your site:

$CFG->theme='classic';

Classic was the other new theme that came with 3.7.

To get boost back, download the 3.7.x code to workstation.  Unzip/Compress the code locally.  Find the 'theme/boost' folder and upload it to your server's moodlecode/theme/ directory.

Check ownerships/permissions on newly uploaded boost folder.

Also ... if using FTP to transfer that folder, make sure you transfer using binary mode ... not ascii ... possible file corruption on upload might occur if using ascii mode.

After uploading the boost folder, comment out the line you added to config.php.   Looks like:

//$CFG->theme='classic';

Server should now use boost, no error or 'white screen of death' ...  and you can investigate in-compatible themes at that point.

'SoS', Ken