On Guild Wars 2, there was a server rollback on the morning of May 11. The game was suddenly reset to the state of Saturday. Many players are now angry and waiting for an explanation.
Update 14:40: The maintenance on the European servers is over. Accounts have been reset to the state of Monday, May 11, at around 11:40 AM.
It may still happen that some players are missing items or there were issues with transactions in-game and with gems. If there are errors, a support ticket should be opened (via GW2 forum).
After the incident, players who played between 11:40 AM and 6:00 PM on Monday during the server shutdown likely lost their progress.
Update May 12 – 10:00 AM: The EU servers are still closed, but at least internally a test is already being conducted. When the servers will go online again is still not clear.
Update 10:00 PM: The servers are still offline. On Twitter, the developers are keeping players somewhat updated.
Update 6:00 PM: ArenaNet has since commented, and they are investigating the issue. At 6:10 PM, the servers were shut down and the game is currently unavailable.
News May 11 – What happened? On May 11, at 9:54 AM, the first thread was opened in the official forum complaining about a rollback.
It seems that the servers were reset to the state of Saturday, May 9, between 7:00 AM and 11:59 AM. Many players are now angry because they used Sunday to play extensively.
Currently, the Super Adventure Box is entering Guild Wars 2, a special event that only lasts for a short time and includes some challenging achievements. Losing progress in it is very frustrating for many players.

No planned rollback
What do we know about the rollback? The error seems to only affect the European servers. This has already been determined by players on reddit. There seem to be no problems on the American servers.
Furthermore, it likely was not a planned rollback, as there was no announcement beforehand and it occurred at a time when the developers likely weren’t actively working on the server.
What do the developers say about the rollback? Currently, there is no official statement from ArenaNet regarding the issue. This is likely because it is still very late at night in North America, where the studio is based.
A statement will likely be published sometime in our afternoon or early evening.

Can the problem be fixed? Many players in the forums and on reddit are hopeful that ArenaNet can reconstruct the state of the game. However, this might be difficult.
On one hand, the rollback could have resulted from an unfixable error, and on the other hand, many people are currently playing and gathering progress again. This would be lost in a rollback to the state of Monday morning.
However, a definitive statement will only be made when the developers speak up. We at MeinMMO have already contacted the developers and will update the article accordingly. Aside from the rollback, GW2 recently made headlines when the third expansion was announced unexpectedly.