I'm sorry to hear about what happened to you at work. It must be terrifying. You may be able to sue them at some point, however, if the store is accusing you of being involved in the robbery, that's more important right now, and the best way to reduce a lot of stress is to speak to a criminal lawyer, and let him do a lot of the worrying for you.
You should NOT talk to your employers, the police, or anyone else about this except for your lawyer. Rightly or wrongly, you are a suspect in a serious crime, and anything you say can be used against you. Once he hears the whole story, he will tell you who, if anyone, it's safe to discuss this with. You should NOT agree to any polygraph without speaking to a lawyer first either. Although a polygraph is not generally admissible in court, if it comes out against you, or it is inconclusive, it could be persuasive enough evidence to get you arrested and prosecuted.
You should also not talk to the police if they contact you and ask you to come in so that they can investigate allegations made against you. These sorts of things can be handled by your lawyer, so just tell the police and/or your employer if they bother you about this that you are waiting to speak to your lawyer first before proceeding.
If you don't know where to start to find a lawyer, you can contact your state Bar Association
. They will provide you with a referral to a prescreened criminal lawyer who they guarantee to be a memeber of the criminal bar in good professional standing. Their referral fee is $50 or less, depending on the state, and it includes a free half-hour consultation with the lawyer. You won't have to hire him afterwards if you want him, but one way or the other, it gives you someone to call in an emergency .
Finally, if there is nothing to their allegations, once that gets established, you can likely seek damages for the harrassment and all that they put you through, and the lawyer you speak to should be able to go into detail with you about that too. But right now, as you are a suspect in the robberies as far as they are concerned, you need to get the air cleared first. Just don't try to do that without a lawyer, as the store's allegations are serious ones.