When I click on multiplayer the servers listed are usually there but sometimes they can disappear.
This rarely happens to me but it can be devastating because it removes all of your servers that you have added.

I've had this problem before but now noticed that in the .minecraft folder there isn't a "server.dat" file but now there is a "servers.dat_temp" file.
A "DAT_TEMP" file

I usually have a folder with a copy of the servers.dat file in case this happens.