We had this happen on one of our sites, the open and close signals work on voltage applied to close, meaning when the wire applying voltage is removed from the signalling unit it sends an open signal.
Its a problem with older kit that uses pins, it relies on a voltage for the closed signal, the intruders got lucky by removing the panel and not removing the panel and the signalling equipment.
We now make sure the signaling equipment on older systems is either mounted within the controls or, has voltage removed to close, so if the panel is removed the signaling doesnt go open.
I think it very unlikely that you will be successful sueing your security company, the system did what it was supposed to do, but it was sabotaged. There is never any guarantee that the system cannot be compromised in some way.