C-x keystroke changed at runtime

[Michele Bert]

I use Emacs (23.3.1 on Ubuntu 12.04) in my daily work, and I can
assert I know it _quite_ well. Allthough today something very strange
has happened.

After more than 1 week that it has been running (continuosly: I use to
leave it always open, even during the night, opening and closing
buffers, creating new frames, even with aid of emacsclient), suddenly
(few minutes ago) the C-x keystroke has changed behavior: whenever
there is an active region, it kills the selected text (just like C-w),
while if there isn't any region, it acts as a prefix for the usual
keystrokes (I can visit a file with C-x, C-f, or save it with C-x,
C-s, or change buffer with C-x, b, for example). But with an active
region it kills it instantly.

I am going to restart the program, but before doing it, I would be
very curious to understand what is happening, so if anyone has a
suggestion about some tests or checks I can do, I would appreciate.

Thanks,
-- 
Mick