If you tried jumping in for a bit of Destiny 2 today and couldn't manage to make the connection, the servers were down for a spot of post-launch maintenance.
You should soon be able to get back in, but before you do, you'll need a 4.795GB update (on PS4 at least) which can be downloaded from now.
The 1.02.01 hotfix patch is mostly aimed at improving server infrastructure and crushing some common errors players were receiving.
It's a good time to do a spot of cleaning up as the Leviathan Raid is just over a day away and will likely attract huge numbers of players.
For info on how to prepare for that, check out our guide on getting to power level 265 in Destiny 2 so you're in the requirements band for the raid.