Could be several things. If you are sure there are no live shells in it (still point it safely), try taking off the forestock to relieve some pressure and try the locking lever again (hard push). If nothing then, and not even a little movement, then the several things part becomes obvious. The firing pin punched through the primer and is stuck (forcing it free will probably bend/break the pin); the firing pin broke or is mushroomed so that it is holding the barrel (could elongate the firing pin hole and gouge the breech face); the locking lever dog or screw broke which will leave the locking lug engaged (disassembly time); there is a burr that developed on the receiver breech face and it captured the shell (it should be able to move slightly - at least enough to get the gun open). Just FYI - I have heard of the locking lug giving folks a problem with the Citori because of a habit of slamming the action shut. Recommendation used to be to hold the locking lever open while closing. I've never done that, and with thousands of round out of several Citoris, never had a problem. But I don't slam em either - just a polite lift and that satisfying thunk/click...