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Check replication process is running on master and slave

show processlist;

If replication process is running, check if slave is lagging behind master.

 start master

If replication process is running, check if slave is lagging behind master.

show master status \g;
show slave status \g;

Looking at output from above commands, you'd know

if slave is lagging behind master.
replication is failing because of some issues replicating a sql on slave.

If replication process has stopped due to duplicate entry in the slave.

STOP SLAVE;
SET GLOBAL SQL_SLAVE_SKIP_COUNTER=1; /*This is to skip the duplicate record*/
START SLAVE;

http://www.redips.net/mysql/replication-slave-relay-log-corrupted/

http://dba.stackexchange.com/questions/53893/mysql-relay-log-corrupted-how-do-i-fix-it-tried-but-failed

https://www.percona.com/blog/2014/10/08/mysql-replication-got-fatal-error-1236-causes-and-cures/