Online servers can be very unpredictable. Games like Destiny 2 can be up and running online one moment and be offline the next. Sometimes this downtime is expected, as in the case with maintenance, while others it can be an outage of some sort.
To find out the status for Destiny 2 servers on Xbox One, PlayStation 4, and PC there is a single location you can head to to check out the global server status to see if Destiny is down for everyone or if it is just an issue you are experiencing yourself. Destiny Server Status Page & Update Status
Find outage information and upcoming maintenance for Destiny 2 at the link above.
Bungie has been trying hard to make the servers stable for the official launch of Destiny 2. Unfortunately, Destiny 2 will see occasional downtime with maintenance as new features are added to the game. The Destiny 2 servers have seen intermittent error codes and outages for some players, but for many the launch of Destiny 2 has been smooth.
Being a mostly online multiplayer game, Destiny 2 is reliant on servers being accessible for players to connect and play.