Original suspension elastomers go bad after 10 years or so. Unless those have been recently replaced, you will likely need to source new elastomers to get the fork working. Alternately, you can convert the fork to use coil springs but this provides no rebound dampening like the slow-rebounding elastomers did.

The fact that the frame came with a rigid fork makes me think it dates from around 1993, give or take a year. Much later than that, it was very unusual for a high-performance oriented MTB to come without a suspension fork. Lack of a permanently mounted rear cantilever cable stop is not a dealbreaker for it being a pre-V-brake era frame.