Site Maintenance Mode:
http://docs.moodle.org/23/en/Administration_via_command_line#Maintenance_mode
In the config-dist.php file of your install there are lines for manually turning on debugging.
// Force a debugging mode regardless the settings in the site administration
// @error_reporting(1023); // NOT FOR PRODUCTION SERVERS!
// @ini_set('display_errors', '1'); // NOT FOR PRODUCTION SERVERS!
// $CFG->debug = 38911; // DEBUG_DEVELOPER // NOT FOR PRODUCTION SERVERS!
// $CFG->debugdisplay = true; // NOT FOR PRODUCTION SERVERS!
For the full story on 'server gone away', see:
https://dev.mysql.com/doc/refman/5.0/en/gone-away.html
Suggest, if you can, increasing max_allowed_packet
https://dev.mysql.com/doc/refman/5.0/en/packet-too-large.html
May as well do that in my.cnf as future versions of Moodle beyond 2.3 will probably have the same requirements.
If on a shared host your hosting provider may have you capped and they don't support 2.3. One way to find out ... ask them! ;)
'spirit of sharing', Ken
Try direct route in browser URL line:
http(s)://yoursite/login/index.php
So including debug parameters in config.php file shows nothing?
One can override Maintenance Mode using the following in config.php
$CFG-> maintenance_enabled = '0';
Uhhh, hope you have a complete/full backup of the site previous to attempt at upgrading.
'spirit of sharing', Ken