This problem has ocurred a few times and is quite troublesome.
After inputting some command (I think each time it was DEplot3D, obviously with some wrong parameter) a popup appears saying that the connection to the Maple kernel has been lost and that I should save the worksheet and restart Maple. After so doing, the saved worksheet seems to be corrupt - it is to be blank when opening it, so I can't figure out what caused the problem and I lose the work I've done since I last changed the name of the file. I've now resorted to changing the name (v1.01 etc) each time I save, which is tedious to say the least !!
I am using Maple 13 classic mode and saving as worksheets (mws files). This is prescribed by the Maple course I am doing.
As a side issue I'll also take this opportunity say that under normal circumstances I always get a message "The worksheet may contain features which are not compatible with the chosen format. Continue ?" to which I always respond "OK", so I'm wondering if I'm doing something else wrong !