How can one enter an airliner without stairs or a jet bridge?
There are no rear entrances these days, but many smaller jets like the Avro-RJ, Bombardier CRJ series or Embraer regional jets have their own air-stair on the main (front left) door. On the smaller planes the door are hinged at the bottom and the stair are part of the inner side of the door, on the larger ones there may be folding stairs stowed beside the door. B737 can have those too, but it's optional, so many operators choose not to have them (and on second-hand aircraft the operator sometimes bother removing them, but rarely adding them back). I've never heard of them on A320 family as it's a lot higher.

You can get always out using the emergency slides and in cockpit they have a rope to climb down through the side window (where it's too high to jump from it) in case the cockpit door can't be used (happens sometimes).

However the only way in is using stairs. It's hard to imagine you wouldn't be able to scramble at least a ladder even at very remote landing strip where the plane has made an emergency landing. Regular landing will obviously not be done to a place without regular stairs (unless the plane has integral airstairs, of course) and if it was emergency landing you need to bring in mechanics and fuel to get the plane out, brining a ladder is not much extra trouble.

Edit: As Kevin noted below, many aircraft have engineering access hatch in or behind nose wheel well. Not an option for boarding passengers, but the crew can use it when they don't have stairs/ladder handy.