If MasterCook crashes when you try to start it up the first thing to do is keep MasterCook closed. Find the MC World.mcw file within the MC TOOLS folder on your computer and delete it. Start up MasterCook. A new MC World.mcw file will be created and hopefully solve your problem.
If you are using MasterCook 11 the Mc Tools folder is within the MasterCook folder which is within the Public Documents folder on your computer.