For some reason Steam disconnected me from a DotA match and activated Big Picture mode, which I now can't turn off. If I click "Exit to Desktop" everything crashes, and every time I start Steam it works for about 10 seconds and then goes into Big Picture mode and refuses to let me use anything else, such as Hammer. The only thing I changed recently was DotA's resolution to fit the monitor better. What happened? There wasn't even a new update today!

Delete your clientRegistry.blob file and try getting into steam again. If that doesn't work then you can move your steamapps folder somewhere temporarily and then re-install steam.

I believe the clientRegistry.blob is the file which controls your saved preferences?

Thanks, restoring the default preferences fixed it. Apparently there's something wrong with the "Start Steam in Big Picture mode" checkbox, even if it appears unchecked it sometimes isn't. I expect VALVe will fix this soon.