Bull queue - Queue was working fine but suddenly, stops working

Question

I have implemented bull queue. It was working fine but now, queue stops working. I checked redis server. These keys are present -

1) "bull:<queue_name>:stalled-check"
2) "bull:<queue_name>:id"
3) "bull:<queue_name>:lock"
4) "bull:<queue_name>"
5) "bull:<queue_name>:stalled-check"
6) "bull:<queue_name>:active"

I am unable to figure out what wrong has happened. Please help in this.