What happened is that you pressed a key or a combination of keys that did it.
But seriously, good that you found your way to the useful keystroke to close InDesign.
If you smack away at the keys again a bunch of times, and can reproduce the strange behavior one or more times, it's a bug, and you can report it here: ADOBE WISHFORM.
I've reported a persistent Quick Apply bug, over the past few releases, a form of which is still present. Namely: if you leave InDesign open for a long time while workiing on other applications, then return to InDesign and then invoke Quick Apply, whether by keystroke or menu, Quick Apply will freeze then InDesign will freeze. This is not the same as the problem you reported.
I left it for a few hours... hoping that the problem would magically go ahead. Tragically, it did not.
In my situation, InDesign is definately not locked up. While that damn "Quick Apply" sits on the screen, I can do some stuff... I cannot enter any text but I can, for instance do the following:
- Hit the Tab key, inserts new rows in a table - I think my cursor was inside the table when this problem began.
- Control - Z undos and most of the other Control keys do stuff.
- Esc highlights the table row temporarily.
- F5 brings up Swatches but I cannot close this window with the mouse. F5 again, closes it.
- F1 web help, F6 color, F7 Layers, F8 info... etc.
That's all, I am confident that my work is saved, so time to close via Task Manager (the only way I guess!).
Forced shut down... back in and everything is good. Whew! Still, that is a mighty strange problem that occurred and no idea how to replicate it.