I have very clear memories of getting in trouble--over and over again!--for burning the congee. I'd be impatient, hungry, inattentive, distracted by any of the innumerable things that distract dreamy little kids--and suddenly there'd be foamy, hissing congee and mad parents all over the place.
Burned congee isn't just inedible--it's virtually indelible. It is the stickiest substance ever encountered in the human experience. It's ruined more pots, both of my parents and my own as an adult (yup, still happens sometimes!) than I want to admit. Even to myself.
So. Learn from my errors and reheat your congee slowly, over low heat. Add enough water to thin the congee back to its original consistency, since the rice has undoubtedly absorbed most of the water while it's been sitting in the fridge. Stir it often, making sure to run your spoon all the way to the bottom to make sure nothing's burning. Be patient. Patience is hard, but you'll save time in the end. Also, you'll save congee. You'll save your pots, and you'll save yourself the aggravation of scrubbing congee goop off your stove.
This is Sunday morning's congee, reheated on Wednesday morning. I was in a hurry because I had an early meeting, but it didn't take too long: I put the congee, extra water, and frozen green beans in a pot, cracked an egg into it, brought it to a low simmer, covered it, and ran around getting ready. When I was done, so was the congee. I put it in a mason jar and took it to my meeting.
(You could in theory also heat congee in a microwave, but I've never done it because microwaves skeeve me out, so I can't offer instruction on that method.)