Help! Update to Joomla 3.4 failed

[NotATechGuru]

Hi,
When I tried upgrading my site (2.5.28) to the latest version, midway through the installation, it displayed an error. When I try to access my website, it displays this error:
Warning: require_once(/home/usedsca1/public_html/pluginsDSsystemDSmobiledetectorDSmdetect.php): failed to open stream: No such file or directory in /home/usedsca1/public_html/plugins/system/mobiledetector/mobiledetector.php on line 113

Fatal error: require_once(): Failed opening required '/home/usedsca1/public_html/pluginsDSsystemDSmobiledetectorDSmdetect.php' (include_path='.:/opt/php54/lib/php') in /home/usedsca1/public_html/plugins/system/mobiledetector/mobiledetector.php on line 113

EDIT: I am able to access kickstart.php but when I'm at "Restoration of site's main database", I get an error saying it can't connect to MySQL

EDIT 2: I've completed the restoration process but when I try accessing my website (both Front end and Back end) I still get an error message.

Front end error message:
Fatal error: Cannot redeclare class JModel in /home/usedsca1/public_html/libraries/joomla/application/component/model.php on line 508

Back end error message:
Fatal error: Call to undefined method JAdministrator::setHeader() in /home/usedsca1/public_html/libraries/cms/html/behavior.php on line 750

Someone please help me . How can I get my website back?

[jcms]

you can login Phpmyadmin and disable that plugin (enabled = 0), then check more bug.
Did you create an backup version?

[NotATechGuru]

you can login Phpmyadmin and disable that plugin (enabled = 0), then check more bug.
Did you create an backup version?

Yes I have created a backup version using Akeebabackup but whenever I try to restore the website, it displays that same message: Fatal Error.

Incidentally, could someone explain how I can disable plugins (as a matter of fact, what plugins do I need to disable) on Phpmyadmin?