gear shifting problem

yeah guyz my 2nd and 3rd are giving me a realli hard time to get into gear all the gears are stiff but those 2 are giveing me more trouble. it was fine until i left it in the cold for 3 days but that shouldn't do anything to it and since then its been a month its being realli sticky and u have to crank it realli hard to put it in gear. before it was realli easy. wht should i do? change transmission fluid or my bushings anyone else have this problem with a 2001 si? any information would be good thanks