I would start by opening the unit and checking for physical damage/corrosion. If things look good in there, I would still do the hot/dry rice thing with it. (even though it's been a while) 1 heat a bowl of rice a few times to dry/extract moisture 2 sure your happy its quite dry 3 submerge all the parts in the warm rice for a day or two That would help extract any moisture trapped under IC's and inside displays.
In my DW, there was a valve that when it was cracked/not sealing kept it from pumping out of the tub. A trip to appliance repair supply with model number, a few dollars later and an hour of replacing it was all it needed to work great. The valve is used to switch from pumping inside the DW to pumping down the drain. Hope that helps.
Starting with a physical inspection of the unit is always a good idea. Look very carefully for missing/vaporised traces, components charred/blown apart, fuses blown etc. Get back to us once you have some more details.
I just want you to understand the risk involved with tinkering with a microwave, from the microwave troubleshooting: "Careless troubleshooting of a microwave oven can result in death or worse. Experienced technicians have met their maker as a result of a momentary lapse of judgement while testing an oven with the cover removed. Microwave ovens are without a doubt, the most deadly type of consumer electronic equipment in wide spread use. Please be very careful if you remove any covers.