If you turn off the main switch (all circuits in the CU are now off) try and reset it. If it won't reset the MCB is faulty and you need a new one fitted. There may be a circuit fault as well.
If it will reset with power off at main switch, and trips as soon as you turn the main switch back on, then you have a circuit failure.
Unless you are familiar with electrical work, now is a good time to call an electrician.
If it will reset with power off at main switch, and trips as soon as you turn the main switch back on, then you have a circuit failure.
Unless you are familiar with electrical work, now is a good time to call an electrician.