"Singer Izumi Sakai has died after sustaining head injuries in a fall from the edge of a hospital building in Tokyo, her management agency has announced. Sakai, a member of the popular group ZARD, was 40."
(Japanese version is here.)
Other articles are here and here.
Sakai made her debut in 1991 and as well as hits with Zard such as Makenaide she also wrote hit songs for other artists:
突然 (Totsuzen) for Field of View
Teenage Dream for Deen
あなたと共に生きてゆく for Teresa Teng
I first found out about Zard through the Best of Detective Conan theme song CD which contained the song 運命のルーレット廻して. I also enjoyed Sakai's cover of 異邦人 with B'z guitarist Tak Matsumoto.
At the time of her death, Sakai was battling cancer but had been planning to release a new album later this year.
According to her official fansite, a service will be held for her 27 June 2007, more details to be announced.