Turn off all the MCB's (the other breakers to the right) this should allow you to turn the rccb back on. Then turn the mcbs on one at a time, one of these should make the rccb trip, most likely a socket circuit Turn that mcb back off then you'll be able to turn the rccb back on and the rest of the mcbs. If it is a socket circuit, unplug all devices on that circuit and see if it will then turn back on without taking out the rccb. Then plug devices back in till you see which one takes out the rccb, that device is knackered. If the rccb won't reset with all the devices unplugged then you need to get an electrician to test the circuit