Disconnected during the final - stage boss fight and the game rolled back

The boss was almost dead when the game suddenly lagged and disconnected. After reconnecting, the game map disappeared, the items in the bag were gone, and the tickets that had been saved for a long time were also gone. What's the meaning of this? Can this game still be played? I worked so hard to be able to fight the final - stage boss, and this is the result.

servers are totaly cripled this morning.
unfortunate