The answer to your question would be the following one: She had been participating in songwriting contests since she was ten: nothing fulfilled her more than music.
A colon means "that is to say". In this case, the colon has been used between independent clauses to indicate that the second sentence explains, illustrates, paraphrases or expands on the first sentence. "Nothing fulfilled her more than music" explains the reason why "she had been participating in songwriting contests since she was ten".
