Koha error

The following fatal error has occurred:

Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (111) at /usr/share/koha/lib/C4/Context.pm line 666.
Compilation failed in require at /usr/share/koha/lib/C4/Auth.pm line 30.
BEGIN failed--compilation aborted at /usr/share/koha/lib/C4/Auth.pm line 30.
Compilation failed in require at /usr/share/koha/opac/cgi-bin/opac/opac-search.pl line 12.
BEGIN failed--compilation aborted at /usr/share/koha/opac/cgi-bin/opac/opac-search.pl line 12.