my citreon berlingo van 1.9D immobiliser got a fault ?

brought a citreon berlingo van 1.8D 1998 only got the spare key not the master key with the key fob had the van a month no problems but today the immobiliser light came on and the van wont start, it turns over but cause of the immobiliser no fuel getting through, without getting a new key how do i bypass this problem