Try to disable all your installed component and plugin on your site. Go to phpmyadmin -> table_extension -> disable it.
Then you may entered your backend back and try reenable your component and plugin one by one.
You may find the issue.
Hi, I have same problem after upgrading to 3.8.0 from 3.7.5
I have a copy of 3.7.5 website.
In 3.7.5 version on my website I disabled all components (plugins, modules, packages) that was'nt joomla native ones,( except FOF, F0F(Akeeba) and few templates) after update to 3.8.0, I got error message in admin :
0 Call to undefined method JVersion::refreshMediaVersion()
If go to System > System information in Joomla! Version field I see:
Joomla! 1.6.0 Stable [ Onward ] 10-Jan-2011 23:00 GMT
If I go to Extensions > Language(s) , I get error:
Undefined class constant 'MAJOR_VERSION'
I loged out and try to login again, and I get error :
Cannot declare class JInput, because the name is already in use in /xxxxxxxx/xxxxxxxx/xxxxxxxxx/xxxxxxxxxxxx/public_html/libraries/fof/input/jinput/input.php on line 39
I can't acess admin area, frontend works fine.