Kirby G4 sweeper stays in reverse mode?

My Kirby G4 sweeper stayed in reverse mode & when in neutral the wheels would spin & it will take of backwards.We took it apart to see if we could see anything obvious,we didn't know what to look for so we put it back together .The wheels didn't spin anymore but still couldn't push it forward. Took it apart again put it back together now the wheels spin in reverse again & can't push it forward.Help please!!!I really like that sweeper.