My mother fallen and her knee ball sliped.now the knee ball is correct position.but she is very scared to walk or fold the knee.is there any problem f?

Absolutely: For bones to "slip" out of position, the complex soft tissues (ligaments ; tendons, etc) which held the bone in place previously were torn. These tissues take months to rebuild plus rebuilding based on body structure after injury, at a below cellular through larger scale. Thus knee will be easier to dislocate again, at least for months. Use powerfully influences rebuilding thus use carefully.