Sayuri, the singer of the group, has had some health problems.
Silver Forest quit mid may this year didn't they? Sad story
Edit: more can be found here: http://www.silver--forest.com/end.htm
Now I can't read japanese, but when you throw that into Google Translate it comes up with this (shortened it):
"Silver forest disbands end of may 2012"
"Physical condition of the main vocalist Sayuri"
"I'm sorry to the other members, Silver Forest without Sayuri is not considered so far"
Silver Forest quit mid may this year didn't they? Sad story

Edit: more can be found here: http://www.silver--forest.com/end.htm
Now I can't read japanese, but when you throw that into Google Translate it comes up with this (shortened it):
"Silver forest disbands end of may 2012"
"Physical condition of the main vocalist Sayuri"
"I'm sorry to the other members, Silver Forest without Sayuri is not considered so far"