If your coffee gets to room temperature, don't microwave it back to hot, instead, put ice cubes in it and enjoy a nice iced coffee.
THE FULL FLAVOR OF COFFEE IS BETTER ENJOYED IF ICE CUBES ARE ADDED TO THE CUP.
I used to microwave re-heat my Lukewarm cup of hot brewed coffee. There are reasons for not just placing it into a microwave oven to re-heat it.
By microwaving a cup of coffee that sits too long without being finished, that will make the cup of coffee hot once again but ... the flavor will not be there. The re-heating of coffee alters the molecular configuration of the certain flavor elements in the coffee and can cause the flavor to become bitter and sour to your tongue's taste buds. I have done this many times and realize that the coffee just doesn't taste good after re-heating it in a microwave oven.
Instead, I added several large ice cubes and added a bit more cream to make it richer and I began to sip on that now very cold iced coffee, and I was very pleased with the rich flavor of the coffee with the added creaminess of the heavy whipping cream or whole milk or even evaporated milk, plus the sweetness of sugar, in just the right amount to make it's flavor almost as sweet and rich as a chocolate smoothie.
Try this just once and I'm sure you will be a believer.
Image Credit » https://pixabay.com/en/iced-tea-dutch-coffee-ice-cubes-241504/