How to Bringing a DB online?

I don’t know why my Couchbase DB went to Offline. I need a reason, why it goes to Offline.
How to bringing a DB online.

I got reference link here

The API documentation for the _online call is available here:

As to why it went offline - the most common cause is that Sync Gateway lost it’s connection to the backing bucket (specifically, the mutation feed for that bucket), and wasn’t able to successfully reconnect after several minutes. At that point the database is taken to an offline state, as it’s not able to serve replication requests properly.

@adamf thanks i will check it.