What is emacs doing? (typo

[ljp]

I mashed some keys accidentally, and now the mouse cursor is spinning
and emacs is otherwise frozen.  There's no indication of what's going
on, and it's not using much CPU. It has been like this for many
minutes.

What is happening?  How can I stop it (short of killing emacs) ?  C-g
does nothing

I'm in python-mode, and I meant to type "C-x f asm"... any ideas for
what keys I mashed that would make it do this?