I just updated from 2.6.5 to 2.6.6.
My items displayed in category view all went to 1 column only - in all categories.
It ignored all the settings for columns and gave me 1 x 1/2 width column for everything

I reverted to 2.6.5 and everything returned to normal

Did you try deleting your cache and resaving your options?

PS. always back up first if you had issues the first time :)

Yes

Check your overrides or your stylesheet for any outdated code.

I downloaded a new copy of k2 - did an unistall/install

and it all worked - I had cleared th cache both times.
Mystery