Bungie rolled out a controversial patch recently (which wasn't even that big, honestly, as auto-rifles are still really effective in PVE), and some users are reporting issues with the game after picking up the download.
The developer notes that you may have to re-install the game on your console just to clear up a bug or two, at which point you'll be able to log in as normal. Don't forget that a new patch is in the works already for buffing some Exotics (Bad Juju was named) and balancing classes.
Bungie Help [Twitter]
via destructoid http://www.destructoid.com/bungie-recommends-re-installing-destiny-if-the-patch-is-giving-you-issues-282586.phtml