Could have gotten a hold of poison, had a head injury or has a disease. Carefully scoop it up and put it in a high walled box or cage. Offer water in a bottle cap and some bird seed. Leave a tissue box inside to hide in and see if it get's better. If it is a wild agouti brown mouse release it outside. Wild mice never have colors such as white, black, tan etc. There is not much you can do for a mouse, it is too small to treat and they either get better quickly or pass quickly due to their high metabolism. Sorry