A white line on the outside part or inside? If it's on the inside just reset it to either a new baby, or to the last important event that your Tama did.
its in the pixel but i tried to reset it and download it but i didn't work for now i just ignore it its not that big but some times it gets really annoying
Ask them if they can repair electronics such as Tamagotchi's, they should be able to help, if not go to your local store.. IDK if they can help but always worth a try.
If its temporary, it will go away in a month or two, the speed will increase dramatically with the tamagotchi off, (With the batteries out.).
Otherwise it'll probably be impossible to fix, and don't go to a store, it'll be cheaper to buy a new one or to DIY it. It's best getting a factory new , brand spanking tamagotchi