The hair on most masks is a synthetic "FUN FUR" type.
This is usually glued on with rubber cement. If this is loose it can simply be glued back down with rubber cement.
Other popular types are crepe wool, or polyester. These types of hair are usually very cottony and can be re-attached with rubber cement which is applied sparingly to the rubber mask, then gently press the hair in place, and if needed, hold 'till dry.
If, However, your mask is a collectors type mask, and the hair is hand laid, (loose hair, glued on and styled by hand) it is best to call the Mask Doctor, and have it professionally repaired.
Not only because laying hair is an art, but different hair is used on different brands of masks.