my question is why can't you move it? Is it that it causes too much pain or you physically can't move it? And is the movement/pain worse or better than before?

Without knowing the answer to those questions or how badly you broke it, I'd be suggesting you pop the splint back on and slowly decrease the amount you use it over the next week with the aim to not need it at all by the end of week 6. I'd also anticipate you may need the splint towards the end of the day after using the thumb all day. I would also suggest you start trying to move it in water to help rebuild your muscle tone in it. Sounds silly but works. If it causes ongoing issues, a trip to the physio may be in order.

HTH

MG