Microwave left on without anything in it except...

Question

My daughter was asked to set the timer for the lazagna for 85 minutes. She pushed the wrong buttons and it melted the plastic splash lid to the glass tray. The microwave shut itself off and it doesn't work at all anymore. Is this a blown fuse or something beyond my ability to repair myself. The small fuse behind the keypad shows continuity. Could it be a fuse cartridge??? Do I need an expert to repair it?? I appreciate any good knowledgable advice.