Same, sprained my wrist/forearm a while back and couldn't rotate it without pain or take any weight palm up. Couldn't even rotate a door knob.

It wasn't until I pushed through with weights, avoiding any underhand grips or rotation, that it started getting better. Doing bicep curls but keeping the thumb up strengthened the forearm to the point where I was back to the weights I was lifting and could then gradually add some rotation.