Unfortunately, you've waited way too long to try to remedy this situation and your chickens are now sporting a brand new habit. It's going to take at least this much time to undo the harm.
First, you need to teach them to come to you. It won't take nearly as long to do this as to undo the trauma caused by the possums. You can accomplish this in three steps in just one day. Start this early in the day as it will take a number of hours.
Gather their favorite treat, perhaps BOSS or scratch or meal worms. Trot out to where the chickens are busy poking around. Right now you just want to teach them a signal to get them to come eventually, but not right at this time.
I use a clicker I bought at PetSmart for a buck. As I toss a few meal worms or a bit of scratch, I click the clicker. You can use a word or a phrase or even rattle the jar with scratch. It has to be an audio cue so the sound carries quite far. Keep tossing the treats and making the audible cue. It needs to be the same cue every time.
Step two. An hour later, stand near the coop and make the noise again. Be ready to back it up with a few treats so the chickens know there's a dividend in running to you when they hear this noise.
Step three. An hour later, you're ready to get them into the coop. You need to stand inside and make the audible cue. If the chickens are too far away, begin this step close enough to them to hear, then back up into the coop. Be ready to slam the door shut when you get them all in. It might help to have someone on the outside ready to shut the door so you don't need to risk having the chickens follow you back out.
Getting them into the coop should be undertaken several hours before dark so the chickens have time to inspect the coop for possums and find there aren't any. Then they will calm down and get reacquainted with their home. You can help by sitting in there and reassuring them calmly.
You will have food and water in the coop and will keep them confined for three days minimum. You will observe them to see if they've become accepting of the coop and have relaxed. Calm them down if any seem upset. Three days may seem long, but they need the time to establish the new habit.
You may need to use your audible cue to get them into the coop at night for a few nights until that habit is reestablished.