Error 500 main site Jommla 3.5.1, what should I do?

Question

Joomla

After the rollback, the following situation arose: the admin panel works, but the site does not!
I turned on the display of errors, but they do not tell me anything. In Jmul full zero.
Strict Standards: Only variables should be assigned by reference in /plugins/system/jblibrary/jblibrary.php on line
26 Notice: Use of undefined constant DS - assumed 'DS '
in /plugins/system/jblibrary/jblibrary.php on line
line 54
Notice: Use of undefined constant DS - assumed 'DS' in /plugins/system/jlibrary/jblibrary.php on line 54
Strict Standards: Only variables should be assigned by reference in /plugins/system/jblibrary/jblibrary.php on line 55
Notice:
Notice: Use of undefined constant DS - assumed 'DS' in /plugins/system/jblibrary/jlibrary.php on line 218
Fatal error: Class 'JFolder' not found in /plugins/system/jblibrary/jblibrary.php on line 219
What should I do about it?

Answer 1

Fatal error: Class 'JFolder' not found in /plugins/system/jblibrary/jblibrary.php on line 219

Fatal error is an unrecoverable error. The reason is here.
Class 'JFolder' not found - The specified file is missing. Put the file back in place. (reinstall the plugin, probably)

Answer 2

I turned on the display of errors, but they do not tell me anything.

it is sad