I can second this happening. I've had it happen at least 40 times in v12. 90% of the time, if I can click the View > Status Toolbar menu item, it bring FileMaker back from the dead and lets me continue my work. Otherwise, I hear a beep with every mouseclick. The Script Debugger can be opened but nothing is running. It's SERIOSULY annoying
There are a few similarities to this bug report, you'll have to judge for yourself if what is reported here applies to your situation or not.
For More Information see: FileMaker enters a modal-like state on editing
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip