My Poulan riding lawn mower will not go in reverse!!! help?

my Poulan PO12530LT 30-Inch 12.5 HP Briggs & Stratton Lawn mower will not go in reverse... I was mowing today and a limb caught the shifter and Pulled it all the way back.... It will do everything but go in reverse... Can someone help me in trying to fix it... Please and thank you