A bungie cord is springy and, when stretched out, will quickly snap back into its original position. A Bungie game imitated that motion yesterday, returning to an earlier state after being stretched beyond its limits.
Bungie deployed Update 2.7.1 for Destiny 2 yesterday morning, but soon after that, the game went offline for emergency maintenance, due to players losing various game currencies. The servers remained offline for approximately nine hours, and when they came back online, players had seen their progress rolled back to 8:30 a.m. Pacific time, losing about two hours of progress before the game had gone offline.
You can view the various progress of the work that Bungie was doing on the forums, as well as the announcement detailing the rollback itself. No further problems have been reported, so hopefully you didn’t take the day off from work to try and play yesterday, like this poor guy who’s certain that Bungie intentionally broke the game just to mess up his plans for his day off.