The glass plates that are in microwaves are made of tempered glass and resistant to heat and shock so it's very rare for them to break but sometimes they do break like mine did a few years ago.
The glass plate breaking can be caused by small nicks or cracks that may happen to the glass when you remove the glass plate or turntable and they cracks or nicks could be so small that they may go unnoticed and then as the glass plate turntable heats up in the microwave it then decides to break.
That's likely what happened to your microwaves glass plate.
You can still use the microwave without the glass plate or you can go online and buy a new or used glass microwave plate through ebay or other online stores.