My hamster is a Russian dwarf hamster. She has always been very active. She is about sixteen months old. When I was went downstairs I noticed that she’s incredibly lethargic, won’t eat or drink. Not even taking her favourite treats. She just wants to sleep, and just slumped on her side with her eyes closed.

I'm sorry she isn't feeling well! A good first step would be to make sure the environment she is is warm enough as hamsters can go into hibernation if their environment is too cold. Making sure she has plenty of access to fresh clean water and food, and fresh bedding may also help. If she still doesn't perk up after these changes, a vet visit to rule out any health issues is best.

I am sorry to hear about your hamster. This behavior (or lack of behavior) is quite concerning. You could try scooping her up with a cub or bowl if you are afraid to touch her. If it is cold where you are you can also put her into a warmer room. In very rare instances hamster can go into a hibernation type state but this is not common at all. If she is still alive/responds to touch but is weak then unfortunately there isn't much you can do at home, so I would try to get her into the vet for an exam as soon as possible. Unfortunately some of these little guys only live 1 to 2 years.