Two levels are a lot. What happens is that the progress that a player makes with their character is not permanently saved on the database server. Two common causes that often work together for maximum frustration are:
1) When you log out, the save does not occur.
2) While playing the game, the periodic save that is supposed to occur every few minutes is not taking place. The process that handles the periodic stopped working.
If you were playing a single player game on your PC, you could create the same issue by exiting the game without saving. Fortunately, it does not happen often. Every online game has this kind of issue. These games would be unplayable due to performance issues if a save was done too often.
It might be nice if the server went down - crashed when the periodic save feature stops working or the save on log out no longer works. That way people do not get frustrated because they are playing the game without getting any improvement to their character.
There is even a term for this situation. It is called a roll back. It can be automatic such as a failure to save your progress on your character. Or could be manual and server wide when the game operator loads a older server state because there is some corruption in the current saved data. There are many other variation beyond these two I have described.
Unless stated otherwise, all content in this post is My Personal Opinion.