The HV circuit on a microwave can have up to 20,000 volts DC on it. It is not recommended for DIY service.
Depending on the model it may have individual HV capacitor, HV fuses, HV transformer or it may be an all in one unit where you may have to replace the whole power module which is expensive.
There are many reasons a microwave fails to heat up, we generally recommend it being brought into our workshop where we would do a check through and cost up replacement parts giving the customer several options.
A deposit of £15 pounds is required which covers costs if the appliance is scrapped, if the repairs are given the go ahead for an agreed price then we will go ahead and the deposit counts towards the final bill.
The appliance will also have had a radiation leak test before the customer collects it.
For future reference posting all the details on an appliance helps us to help you.
Make, Model, Serial Number, Product Number (PNC, FD, Etc).