We have a replication process on going.
That is a Master server is replicating its data to a Slave server.

But all of a sudden the replication is not running anymore.
That is when I issue ./show_slave_status all I see is:
Seconds_Behind_Master: NULL

which of course means that Slave is not replicating from Master.
And of course I have shut down and re-started Slave, but no good.

So do you have a suggestion as to what the problem is and how I fix it?

FYI, when I run mytop on this mysql server I see:

Id User Host/IP DB Time Cmd Query or State
-- ---- ------- -- ---- --- ---- ------
11294 root localhost test 0 Query Show full processlist
8671 system us 303499 Connec Has read all relay log; waiting for the slave I/O thread to update it