I've only had this problem for a couple weeks but all of my server data on my game has been disappearing. It first happened a while ago when I logged on and it was gone. I ended up making a minecraft forum post that didn't end up getting any replies. The weird thing about it is it deleted all server data such as icons, names, and IPs but it never deletes the server on the top of my list (of 50<) which is the official skyblock server by the original creator(s) of skyblock. After having it happen recently (an hour ago) I now have more information on the subject.
When it happened again today it started with my computer crashing twice (blue screen that says something has gone wrong (the one where you don't lose all memory on your computer)) both times seem to have been caused by minecraft itself. The first time, I had minecraft open for a while and when I went afk on a server for over an hour I came back to a frozen game. Unable to quit, I simply pressed the 'x' button in the top right corner of the window and my computer crashed. The second time, I had been playing for a while and when I decided to take a break and went to hit the 'quit' button on the main menu (I was able to exit the server no problem) the game froze for about a second and the computer crashed once more. Once I finally came back I found that once again all my server data (besides the skyblock server) had been deleted.
I am not using an official Minecraft client instead I have been using the SquidHQ launcher which only bypasses the EULA agreement in order to access blacklisted servers. As far as the client itself it is vanilla and there is no mods gameplay or aesthetic mods that come with the launcher.
File: When I asked someone if they new how to fix the problem, they asked me to find a folder that was called server.dat which did/does not exist
Does anyone know how to prevent this problem or how to possibly get my servers back? Any help on the matter would be greatly appreciated!