Honestly it looks fine to me - fat, definitely, but my green neons often rest in the water at a tilted angle. Normally it's when they're looking for food below. I think of it kind of like the fish equivalent of a cat waiting to pounce. To me it looks well fed, but not worrying. If it's a big change in behaviour/ there are other signs of distress it is worth investigating further. Do you know how long the heater was off for?
Others may be able to help more!
Observe the scales. If they look normal, then everything is probably fine. If the scales are sticking out, then the fish has an infection.Do you just mean it's very plump? Or is there something strange with the swimming position?