I've lost my crowbar!

[Serena]

I guarantee you that the crowbar would not work, even if you had it in your inventory. The window may be glitched in some way, but it is intended to be broken by thrown force. I can't imagine why it's not breaking, a chair or rock from the nearby caved-in hallway really should work.

Try loading an earlier save, perhaps a script did not trigger - normally the game makes you look at the window briefly as glass tinkles, as a hint that the window is breakable.