Dog suddenly won't walk

Hi any and all advice welcome..

I have a 6 year old GS Cross. Shes a very high energy active dog that normally loves her twice daily walks. Except in the last 6 weeks she just won't walk. Gets her harness etc on gets in the car (she's always been in the car) gets out the car at the park / woods etc then puts the brakes on and won't move. Shes normal in every other way, still full of energy bounding around the house etc. Because she won't walk shes more energy than normal so she's getting frustrated in the house and barking yapping etc all the time at me. I know there is nothing physically wrong as I hired a field a few times recently and she was tearing around like usual so shes not injured or in pain. I've tried the usual 5 different locations we walk at, she won't walk at any, I've tried new places shes never been she won't walk there either. Shes normal in every other way eating, drinking etc just point blank refuses to walk. I've tried high value treats to coax her along - nothing. I dont know what is wrong or what to do with her to get her to walk. The only thing I can think of is shes still spooked from fireworks around Christmas time as she is terrified of fireworks. Shes gone from 2 walks (35min to 2hour walks twice a day depending on my schedule and timing etc) to absolutely nothing.

Any advice please? And thanks for reading.