It is silicone rubber, and you do get repair kits for those keyboards which have that membrane in them as a new part, as the silicone wears out and the carbon printed tracks go high resistance. however try first cleaning with a lint free cloth and gentle rubbing of the gold PCB and the black printed side with isopropyl alcohol, nothing stronger than ethyl alcohol 90% or pure isopropyl alcohol, to remove any deposited film that prevents contact. Light rubbing, as that carbon ink will be very fragile now.