My dentist wouldn't treat me saying I owed money, but I find out the dentist actuly owed me money. I was overcharged and was due a credit. I suffered for 3 months and was harassed weekly by a collection agency. I asked to get my credit back and was told the dentist would no longer treat me. Telling me to take her to court to get it back.

Of course you can sue. You pay a fee, fill out some papers, prove your case with copies of proof of payments, and send it in. You can either have a court date or the defendant can decide to just pay you (including the filing fee you paid, often about $50).
Be sure your proof is good and well organized or you might lose.
You won't get anything for suffering for 3 months. You should have gone to another dentist and sued sooner.
The dentist (if wrong) will be ordered to fix your credit rating.

Why is the dentist being so nasty?

Anyone can sue over almost anything. The main question is whether you can win a suit. The secondary question is whether its worthwhile to bring suit. The answer to both those questions in your case is NO, in my opinion. Without knowing why the dentist thought you owed money, we can't tell if he was negligent or not. If he wasn't then you will lose. Even if he or his accountant was negligent, you will have a hard time proving damages.

As far as not seeing a dentist for 3 months, there had to be other dentists you could have seen. So that's more on you then the dentist.

While it doesn't cost much to sue in small claims, the time and effort you expend will be significant. And, like I said, I don't think you will win.

If the dentist actually owed you money, you can sue for that money back.

You can not sue, for not being treated, you just go, to another dentist if you needed treatment.

I agree with everyone else... you basically shot yourself in the foot for not going to another dentist. And its quite common to refuse service when money is believed to be owed until its settled. There are plenty of other places to go in the mean time. If it was as bad as you claim with "suffering" you would have sought out treatment elsewhere.