You have a significant electrical fault. Might be cable damage, e.g. if mice have gnawed it, or if someone has driven a nail into a wall, or if the cooker has been pulled out for cleaning and pushed back trapping the cable underneath.
You could start by turning off the big cooker switch in the kitchen, and seeing if that enables you to turn on the MCB without tripping the RCD. If it does, then the fault is on the cooker or its cable, so address it as an Appliance fault. If not, the fault is between the consumer unit and the cooker switch so address it as a Wiring installation fault.
If the fault is at the cooker end make sure all the rings and oven are turned off and see if that helps. If so, you can isolate the fault to one part of the cooker. In that case, unless it is the Oven heating element (which often goes wrong and is reasonably easy to change) I would be inclined to call in an appliance engineer.
BTW as cookers often have a certain amount of minor earth leakage, it is usually considered good practice to have the cooker circuit on the non-RCD side of the CU. This decision is easier if the cooker switch is one without a socket.
