[Angstrom]: /opt is a file


yesterday I ran Angstrom without any problem and I shut down the system.
Today, I've just discovered a strange behavior: the /opt folder isn't a
folder anymore!

If I type: cd /opt it will screams "can't cd to /opt".
From the GUI I can see that opt is a file in / folder of several MB...
But a file, not a folder!

What could be happened? And how to fix?

apparently some data was not written properly or incorrectly during
fix is: run fsck