Considering how red her eyes are I'd say she had a bad day and had been crying a lot. Taking your quote into consideration it's very likely that Lyra saw something that made her speechless. This could be any number of things. For the sappy people, myself included, Bon Bon could have been setting up a kind of party to cheer her up. For the people who like sad theories and stories she could have walked in on Bon Bon with somepony else! Le' gasp! Then for the people who like dark and grim stories she could have opened the door to see Bon Bon having hung herself. Ugh... I can't believe I even said that last part..
pictures worth 1000 words. I'm guessing she walked in on her suicide, hanging from the ceiling fan, still slowly swaying back and forth, with her lifeless, pale face and her bleached eyes still wide open starring back at Lyra.