Florence Welch (@flo_tweet) herself provided most of the details in a series of tweets today:
July 11, 2012 @ 11:15AM : hello everyone, thankyou so much for all your messages of support, yes its true..it finally happened, I've lost my voice..:(
July 11, 2012 @ 11:18AM : I've sustained a vocal injury and been told i cannot sing for a week.., seriously i felt something snap, it was very frightening.
July 11, 2012 @ 11:20AM : unfortunately this means i will not be able to perform at Benicassim and Optimus alive festival this weekend..i am so so sorry..Florence used the words "felt something snap" which is pretty classic for a vocal hemorrhage. For those who are Adele fans, something similar happened to her as well in November which mandated a prolonged period of voice rest (months) and even resulted in vocal cord surgery. (read more)
I am guessing that Florence had a blood vessel on her vocal cords that was causing minimal vocal impact. But with the "snap," the blood vessel likely ruptured causing instant vocal decompensation whereby the vocal pitch suddenly becomes much deeper and if bad enough, even complete voice loss as it appears to have happened to Florence.
Florence Welch told not to perform "to avoid permanent damage" to voice. CBS News 7/11/12