Please help!

I have a leghorn pullet about six weeks old. Last night she was acting as though she were drunk. Was brought to vet this morning and he seems to think its neurological issue. She was given a cortisone shot. She has no broken bones and hasn't been physically hurt to my knowledge, now she won't stand or walk. Looks like she has an equilibrium issue, to me. I am very new to the backyard chicken world. She is one of my first three. Thank you, in advance, for any help.

Welcome to BYC. Although Mareks disease can be a cause of loss of balance and lameness, dehydration and coccidiosis are more common causes. Typical symptoms for coccidiosis are lethargy, weakness, hunched posture, not eating, and runny droppings with mucus or blood. It is very common in chicks that age. Corid (amprollium) is the treatment.

In Mareks disease, the symptoms can vary in every case, but it can start with paralysis of one or both legs, wings, or the neck. Wasting can occur from not being able to reach the food, and their are immunity problems.

Hopefully, this is dehydration or cocci. I would put some SaveAChick vitamins with electrolytes or another brand, into the water, and make sure the chick is drinking well. It won't hurt to start Corid in the water for 5 days if you suspect coccidiosis, but don't give vitamins at the same time, wait until after Corid treatment. Dosage is 2 tsp of the liquid Corid, or 1 1/2 tsp of the powder per gallon of water for 5 days. It is found in the cattle medicines in feed stores.Here is a good link to read about coccidosis and another 2 about Mareks disease:
http://www.the-chicken-chick.com/2012/12/coccidiosis-what-backyard-chicken.html
https://extension.unh.edu/resources/files/Resource000791_Rep813.pdf
https://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq

Thank you for the advice. I am heading to tractor supply right now to pick up some CORID. Her droppings aren't bloody, however.
I've been giving her 3 ml of Pedialyte every two hours as it doesn't look like she can drink on her own. I've added some SaveAchick
electrolytes to her water as well. Should I inject some in her mouth also?
Is it my understanding that if it's Marek's, then there really isn't anything I can do?
Since she's unable to drink on her own, should I mix the corid in water and inject it in her mouth?
Thank you again for your help. We've become attached to Pickles over the past 6 weeks and the family and myself are pretty worried.