If the silver screw knob or shaker lid looks to be locked on the grinder and you can’t get it off. Rust between the two metal parts might have just occurred. Carefully separate the salt part of a rock salt grinder while replenishing to eliminate this or when using a new grinder.
When you fill it in, salt gets stuck in the junction of the pole and creates rust. Gently remove the salt part and replace it with prepared rock salt after refilling. Put a tiny amount of cooking oil into the pole string (e.g., olive or vegetable oil). Remove the salt part and shaker top, then tighten the knob.