Thank you for your response. The plate is made of china, and was not used at all in the previous at least 72 hours.
Shalom – If so, the law is as I wrote – "the food is kosher, as well as the microwave and the meat cutlery. The plate needs koshering." A china plate can generally not be koshered. If there is no great loss here, it would be best to dispose of the plate. If however it is part of set, or has great value, one could rely on the opinions who consider microwave cooking as a lower level than cooking in a regular oven. If so, the plate could (according to some) be koshered by first waiting 24 hours from the time of the microwave use, then immersing it three times in boiling water. I hope this is of help to you. Blessings.