Su-15 (as well as all Sukhoi interceptors since Su-7) had a completely irreversible control system with no manual override. This means, 100% of torque was produced by hydraulic boosters, and the force on the control stick was simulated with a special variable spring loading mechanism.
So, a theoretical total hydraulic failure would result in loss of control, while the stick would move 'normally' - just without any effect.
Su-15 had 4 hydraulic systems, with 2 of them used for flight control. The most anticipated reason for dual failure would be loss of the engines; for that, there was an electric pump in one of the systems. In case of control problems, the pilot would check and ensure that the emergency pump was on. This pump provided only a limited amount of control (and for a limited time).
Now I'll have to challenge the premise a bit. First, Su-15 was an interceptor, and low-altitude flight (even training) would be quite unusual for it (though not impossible). Its lowest altitude of intercept was 2 km, and the main emphasis was on intercepting higher altitude targets. Its radar was unable to track targets on the earth surface background. It was not meant for terrain following, so even if it was flying at a low altitude, it would be fairly clear of immediate obstacles. Which gives the pilot at least a few seconds to react.
Second, the pilot would, of course, eject. Even though in emergency situations (esp. unanticipated emergencies) people do all sorts of things, pilots, and esp. pilots of aircraft with irreversible control, know that loss of control = eject. It's pointless to 'fight' with the stick. However, as I said, in all likelihood there would be a few seconds to think. In most situations (like the engines failure) you won't have an immediate loss of control: the failure is fairly gradual. This would see the pilot trying to ascertain what was happening. Difficulties with control at low altitudes often require reducing the airspeed.1 But a catastrophic sudden loss of control is a clear signal to eject.
1 By the way, early Su-15 had pitch control problems at transonic speeds at low altitude due to insufficient booster torque. It felt like no response (or poor response) at higher G manoeuvres. This things could be resolved by reducing speed.