I have a young mare who has never had issues loading. Usually calmly walks right in. I recently brought her to a 4 day clinic (I live close and hauled her in and home each day). The last two days were very long days and she isn't used to that. I tried loading her the day after and she refused completely. She'll walk up to the trailer, lean over and look around calmly, but won't get in. I believe her having to work so much at the clinic soured her to loading. What's the best way to overcome this issue? She's very sensitive but stubborn and I don't want to traumatize her when it comes to loading. 

Do you tie any kind of a hay bag in there even for short distances? I think your probably right, just associates trailer with hard work after the clinic. Just have to make it a happy place again. If you can have somoene help you load, then take her for a ride with no work amd see hopw she does. Thats a tough one! 

Just start from square one. Pretend she's never loaded before. Retrace your training steps.

What has worked the best for me is to lead them up to the trailer ... at some point, they will stop. You maintain pressure on the lead and you wait.

And wait.

And wait.

And wait.

Do not budge. Do not pull harder. But you do not release the pressure on that lead rope until you get the slightest  forward motion. But once you get the tiniest forward motion, you immediately release the pressure (reward them) and maybe even go over and rub on them and pat them. Wait about 10 seconds (let the idea "soak"). Then try again.

You will need to have a lot of time on your hands. And you will need to have patience. I've had this take 2 hours before for horses.

If your horse tries to go backward, make it very, very uncomfortable. Back them up. Fast. Long. If they want to stop backing up, make them back up some more.

And they try again.

The key is that you aren't making a big deal out of anything. Your body language is quiet (unless you are making them uncomfortably backing up, then have some energy). You are keeping the trailer a peaceful place so long as they are either standing still or moving forward. You arent' running them in circles and you aren't getting them frazzled. You are simply asking them to follow that lead rope and move their feet forward.

SIMPLE. (but oh so hard!!)

Good recommendation above. Another thought if the horse has always loaded easy. Check for a wasp nest in the trailer. I have seen this a few times and it will sure make a broke horse not want to load again. I have not seen a horse refuse because he had a long tiring day previously. 

Failing this, starting from scratch and teaching to load properly would be the way to go. Some horses need a refresher course at some point, but if he has been walking right in and will not now, somethng has changed.

Mine are all taught to lead as babies with butt ropes. They all try you once in awhile. I put a butt rope on mine and the still think I am bigger than they are. Another thing that works well for me is walking the up to the trailer. From inside the trailer pull on them really hard then immediately give them slack and usually they hop right in. I did have one horse that I had to run a long rope through a tie ring in the trailer then get behind them and smooch. They may set back but when the realize that nothing budges, the getting. Then I have the self taught program for beginners. I back the trailer up to the round pen and feed in the trailer. 

Sore hocks

If you don't have time to mess with it, use a lip chain.