You open the door and your dog plants their feet. Or they get in, then shake, drool and stare at the floor mat. A month ago they hopped in without a thought.
This is not the barking version
A dog who barks the whole journey is usually excited or frustrated. A dog who will not get in is telling you the car has become something to avoid. The handling is different, and it is slower.
Rule out feeling sick first
Travel sickness looks a great deal like fear, and a dog who feels sick cannot be trained out of it. Drooling, lip licking, repeated swallowing and vomiting all belong in a call to your veterinary practice.
Make that call before you start the work below. Travel sickness and travel anxiety are treatable, and treatment is sometimes the whole answer.
Stop loading your dog in for 2 weeks
Every lift into the boot teaches your dog that the car ends with being handled. For the next 2 weeks, drive only when you have no choice.
If you do have to travel in that window, keep it short and calm, and give your dog a chew when you arrive. Then go back to the plan.
Week 1: the car goes nowhere
Park somewhere quiet and open the doors. Sit on the sill with a bag of food for 3 minutes and let your dog do whatever they like.
Scatter 10 pieces of food on the ground beside the open door, not inside. Let your dog eat and walk away again.
When your dog puts 2 feet in on their own, feed 3 pieces there, then let them get straight back out. Getting out is part of the practice.
Twice a day, 3 minutes each. Staying below the point at which your dog will not eat is the condition for learning near something they find frightening.
Week 2: the engine, then the end of the road
Once your dog gets in and stays for 30 seconds, close the door for 5 seconds and feed through the gap. Then open it again.
Add the engine running for 30 seconds, parked, with food. Then engine off, then out. Do that for 3 days.
Then drive to the end of the road and back, and let your dog out at home. No destination, nothing thrilling at the other end.
If your dog refuses at any point, the last step was too big. Go back one step for 3 days.
Short repeatable sets, kept below the point where your dog gets uneasy, are how calm is built. One big hopeful journey undoes a fortnight.
What not to do
Do not lure your dog in with food and then shut the door on them. That teaches your dog the food is the trap, and you lose the food as a tool.
Do not scold, push or drag. Punishment-based approaches are associated with fear and avoidance, which is the exact thing you are working to reduce.
Safety note
Get help now if your dog panics in the car, or urinates, vomits or cannot settle at all. That needs a veterinary examination and a qualified behaviour professional rather than a plan from a page. Sudden behaviour change warrants a veterinary check, and pain often presents as a change in behaviour.
How long this takes
Plan on 4 to 6 weeks, and longer if there was a crash or one very bad journey. Nobody can promise you a dog who enjoys the car. A dog who gets in and lies down is the realistic aim.
Tonight's version, in one line: open the doors, sit on the sill for 3 minutes, and feed on the ground outside.