The hair on most older masks is a synthetic "top wool".
By this I mean it IS wool, and it is shorn from the top of sheep, and cleaned, and dyed to the desired color.
This is usually glued on with contact cement. If this is loose it can simply be glued back down with more contact cement.
Other popular types are fun fur, or spun polyester. These types of hair are usually very cottony
or come on a sheet,and can be re-attached with contact 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 on the Mask Doctor, or have it professionally repaired.
Not only because laying hair is an art, but different hair fiber is used on different brands of masks.

  Gluing Top Wool Hair, in layers on a mask

                                                                            Sometimes a wig can be added…