Well, if you have no power, the relay is most likely always energized. First thing, measure across it's coil. If there's no voltage, then the relay is probably bad. If there is voltage, something has failed in the trigger circuit.
If you look at the circuit, you'll see that L1 is the current sensor, it's what starts the triggering process. However, if nothing is connected, it's unlikely to be that circuit as the only way it outputs voltage is by induced current. I'd start working my way through U1b and U1c first and see if they're charging up C5, that would trip the relay. If so, I'd consider maybe replacing the LM324. If that circuit isn't tripping, then I'd look at the transistor Q1 next, does it have a constant positive voltage on the base of more than .6 volts? If so, perhaps something is amiss with the latching circuit. Q1 could also be shorted.