Quick answer
The fault causing it to trip is still present. Switch off all circuits under it, try resetting, then switch circuits back on one at a time to find the problem.
If your RCD will not reset, the fault that caused it to trip is still there. Do not keep forcing it. Follow these steps: 1) Switch off all the smaller switches (MCBs) under the RCD. 2) Try resetting the RCD — it should now stay on. 3) If it still will not reset, the RCD itself may be faulty or there is a fault between the RCD and the MCBs — call an electrician. 4) If it resets with all MCBs off, switch them back on one at a time. When the RCD trips again, the last MCB you switched on controls the faulty circuit.
Need more detail? Choose a view: