You are probably also wondering what might have happened. One of the most likley possibilities is that Mouse was injured. Something may have frightened him and he ran into the side of his cage or a toy. That can result in head or spinal injuries. If this is what happened, only time will tell if he can recover.
Diabetes can also lead to lethargy and even a coma. Diabetes is not unusual in hamsters.
There are no diabetes medications for hamsters, so diabetes is usually treated by changes in diet. Diabetic hamsters are fed a quality seed mix, alfalfa, carrots, cooked potatoes, and boiled egg. You can read more about diabetes in hamsters here:
If Mouse is a Campbell's dwarf, it's even more likely that he has diabetes. There is an inherited form in the Campbell's, which usually strikes young adult males. This link will tell you more about that:
There are other less common possibilities, such as an inherited form of heart disease called cardiomyopathy. If you can check inside his mouth or the pads of his feet, look to see if the skin there is white or blue colored. That means blood is not being circulated well. That is often the result of heart disease or anemia.
Finally, make sure he didn't get too hot or too cold. Both can lead to these symptoms.
From all these possibilities, you can see why a vet visit is needed. The vet can listen to Mouse's heart and lungs, do a blood test that would diagnose diabetes, and check for injuries. If you have more questions, just let me know. I hope Mouse will be able to recover from this.
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