Check your bearings! sounds to me that what happened when you backpedaled was you tightened the axle against the bearings. That would defo explain why the wheel wouldnt move again until you pedaled forward. i would bet my bottom dollar you damaged your bearings. check the races too. if the bearings put scratches or divots in the races, then you will end up needing a new wheel. that hub is no good once you scratch or gouge the races, because then you have soft spots, and even under normal use, the bearings will continue digging further into the races, and eventually dig so far in they will get stuck. when that happens, your wheel will lock up. if your going 20 or 30 mph, your bike will stop, but you wont. put that coaster brake back on. if you scratched your bearing races, you need a new wheel. if you damaged bearings, you need new bearings BEFORE they scratch the races. if you dont replace bad bearings, they WILL scratch the races, and then youll need a new wheel anyway. most likely you didnt scratch the races yet, but check anyway. put the brake back on and replace the bearings.