I'm so sorry for your bad grooming experience.
The reality is that most "store" groomers don't have a lot of experience with Shelties. They are used ****-tsus and golden doodles, and yorkies and morkies, etc. That means....mostly shaving. Its the easy way out. Matting? No problem, just shave it away! That's what most groomers do with these designer dog mixes.
I'm blessed that, working together, my store groomer has self educated herself to avoid the shaving pitfall. She now has a Sheltie owner following which consists of about a dozen clients. now. Including my own, of course.
All to say.....shaving is not forever....maybe.....sometimes shaving can result in weird hair growth....ie. the undercoat grows back fine, but the guard hair may not. My Bacca had his butt shaved (I'd asked for a poop chute only) and the guard hair never grew back properly after that. His butt hair was wonky for years after.
Regardless, it might take as much as a year to grow back, if it does at all. I don't want to discourage you, but I do want you to have ammunition to complain. And I most definitely would. But be 100% sure of the conversation with the groomer. If the dog was matted, the groomer may have told you there was no choice but to shave. There are most definitely limits as to what a groomer can do. If you gave her carte blanche, then you won't have much to stand on.
You will get get different opinions on the forum about shaving your dogs. Many in the southern territorities do it and claim no issue.
I'm on the opposite side which says NEVER! Personal choice.
All to say.....when you are starting with a new groomer, you really need to ask a lot of questions.....but if you are new to Shelties, you may not know the questions to ask.....Hmmm.....seems like a new thread on Sheltie Grooming 101 ......What to tell your groomer????