There is no power to the unit at all circuit breaker not tripped

If you are not sure in ANY way about using this microwave BUY a new one. Simple as that!!
As long there where no sparks, strange noises or strange smells you should be OK to use it. Most modern microwaves have safety electronics built in that can act as a safety switch.
You can buy 'Plug In' adapters that are designed as a safety circuit breaker. Try searching for "Plug through RCD adapter". These 'Trip' if there is a power surge stopping the power to the item plugged into it. I have one for when I use big power tools.

Generally microwaves should be on their own breaker. Hard wired ones are required to be by code.

If yours has a plug - make sure the plug is original with the unit, not added later or spliced onto hard-wired power leads.

Plug into and outlet rated 15A as the only thing on a 15A branch circuit and try again. If it still trips, try a 20A line and circuit. If it trips that, trash the microwave and buy a new one. Nothing is meant to be repaired inside.

That unit shows that it takes 15 or 20 AMPS to run. What is the breaker amperage on the circuit where it is plugged in? Is the breaker a GFCI? You can tell by looking for the two buttons on the circuit breaker or the outlet.

I would suggest you check the door switches inside the microwave oven just below the keypad, rectangular in shape. The switches are normally open or closed, set meter to ohms, unhook lead from one side and test with door open and closed. If readings are the same both ways the switch is defective. "check all switches"

your power transformer for the magnetron could have a shorted primary power winding,the door safety interlock switch could be mis-adjusted,this normally blows the internal unit fuse,a shorted high voltage capacitor,or diode,a bare wire inside the unit,more than the microwave on that circuit or a weak circuit breaker in the house power panel

The circuit is probably a 20Amp circuit (see the number stamped on the circuit breaker switch). Most likely, it is the same circuit as the refrigerator, crock pot, window A/C unit, etc....

Also, if you are using an extension cord or a power strip or a surge protector. Those may not have a high enough power rating.

One other possibility (not likely) is that the circuit breaker is a 15Amp and the microwave is 1500 watts.

What else is cut off when the circuit breaker is tripped?

Solution is to use a different outlet for the microwave, or turn off any other applicance in the same circuit (except the refrigerator, of course).

Hope this helps!

This could be the breaker at the panel or the fuse inside the microwave.I would check the panel box for any tripped breakers and then the fuse inside the unit.

Check what other appliances are on your circuit. What goes off when the breaker is tripped. You may need to relocate the microwave to a different circuit. Try to stay away from the refrigerator or AC circuits.

Fan turned on after cooking because a thermostat got to hot and tripped turning on the fan to try to cool the unit.  The unit went completely dead when thermal fuse tripped which is located above the cavity on the left side.