I have 2 servers in Replication mode, of course one is Slave and one is Master.

2 questions:

1- If the Slave is showing this value:
Seconds_Behind_Master: NULL

does this mean that Slave is in sync with the Master or it is not running?

2- If it is not running, and given that I issued the command to restart it already, then what should I do to restart it? To trouble shoot any problem?