a strange problem: wordpress theme doesn't show up

wp 3.1

before:

I cl开发者_StackOverflowick themes, twentyten theme is one of Available Themes

after:

I rename default theme folder twentyten to yoyo, and change some value in style.css, then click themes link, yoyo doesn't show up.

if I rename yoyo back to twentyten, then twentyten show up again.

PS: I check permission with yoyo, it is readable to everyone.