Replaced the fuse and it immediately blows. My wife cooked several large potatoes got it ral hot before failure

See the steps and eliminate short circuit use the common sense: God bless you
No Lights or Power, Unit seems Dead First check is the main fuse of the unit. On countertop units, you will need to take the shell off. The fuse will be close to where the cord comes through into the unit. OTR units, this main fuse is usually behind the control panel or possibly behind the vent. Usually the fuse is obvious if blown, otherwise test with an OHM meter, if open, replace fuse.
Some brands will also have a thermal cut-off fuse in the incoming voltage line to the control board. Older Maytags and Samsungs have this fuse in it. This thermostat looking fuse is usually above the oven cavity somewhere under the outside shell. Sometimes you can get to it through the vent of OTR models. Test with an OHM meter, replace if open.

However if after replace the fuse the problem remain, you must test: a) power cord for short b) outlet wall for ground improperly c) HV transformer faulty d) Rectifier HV faulty e) Magnetron faulty. d) power supply faulty replace or repair filter diode, resistor. etc Or call some experimented service tech for troubleshooting.