Buy a manual, and take the clutch cover off. Something in your shifter mechanism is broke or not in proper sequence for shifting. It may have fell over and popped the shifter shaft out of position. There is a shifter star on the shift drum, that needs to be set in neutral, then engage the shift shaft centered. There are 2 detentes that help hold the bike in gear, they may be loose or missing. Its worth checking before the cases get split. The worst part is the clutch basket really needs to come off for access to the shifting parts. Total cost, 1 clutch cover gasket and a shop manual. Vintage Bob