Check the core/cache/logs/error.log file.
Also, look at the content of the /manager/index.php file to see if it has changed.
[2016-02-04 10:49:38] (ERROR @ /manager/index.php) Error HY000 executing statement:
Array
(
[0] => HY000
[1] => 2006
[2] => MySQL server has gone away
)