I can only open my

if the car has been stood for a long unattended period then yes the door rubbers could have stuck the door closed,or the central locking solonoids have have stuck but you did say the knobs go up and down so iam not sure about this one.This is another hands on type thing where i need the car in front of me,but one thing you could try is get a nice big piece of cardboard,climb in back of car and with a nice dolly bird as an assistant to hold the door handle up from outside place cardboaur over trim lay on seat in a compromising position and give a sharp kick with both feet on the end of door furtherest from the hinges.have to be explicit in case your a colonial cousin....if this doesnt work then call a mobile mechanic as this is definatly a hands on type of job.with lots of variables depending on the model of jam jar